Don't generate matrix jobs for Swift versions which aren't supported by the package. (#3415)

Don't generate matrix jobs for Swift versions which aren't supported by
the package.

### Motivation:

Matrix workflows should not generate jobs for Swift versions < tools
version, doing so wastes compute and adds noise.

This means that:
- New repos don’t need to manually opt-out of old Swift versions
- Repos which have dropped an old Swift version (during a migration
window) won’t have failing tests until it’s turned off centrally.

### Modifications:

- The generate script now looks for `Package.swift` and
`Package@swift-*.swift` files and uses the lowest tools version in any
found file to determine the minimum supported Swift version
- Any Swift version lower than the minimum is disregarded and no jobs
are generated for it
- A new parameter is added to the workflows which allows the minimum
Swift version to be set manually (`minimum_swift_version`)

### Result:

By default in this and downstream packages we don't generate matrix jobs
for Swift versions which aren't supported by the package.


An example of this working:
https://github.com/apple/swift-nio/actions/runs/18711362977/job/53360387720?pr=3415

Author: rnro

.github/workflows/benchmarks.yml

@@ -76,6 +76,11 @@ on:
         description: "Environment variables for Linux jobs as JSON (e.g., '{\"DEBUG\":\"1\",\"LOG_LEVEL\":\"info\"}')."
         default: "{}"
 
+      minimum_swift_version:
+        type: string
+        description: "Minimum Swift version to include in the matrix. Leave empty for auto-detection from Package.swift, specify a version like '6.0' to override, or set to 'none' to disable filtering."
+        default: ""
+
 jobs:
   construct-matrix:
     name: Construct Benchmarks matrix
@@ -101,6 +106,7 @@ jobs:
         env:
           MATRIX_LINUX_COMMAND: "curl -s https://raw.githubusercontent.com/apple/swift-nio/main/scripts/check_benchmark_thresholds.sh | BENCHMARK_PACKAGE_PATH=${{ inputs.benchmark_package_path }} bash"
           MATRIX_LINUX_SETUP_COMMAND: "swift --version && apt-get update -y -q && apt-get install -y -q curl libjemalloc-dev && git config --global --add safe.directory /$(basename ${{ github.workspace }})"
+          MATRIX_MIN_SWIFT_VERSION: ${{ inputs.minimum_swift_version }}
           MATRIX_LINUX_5_9_ENABLED: ${{ inputs.linux_5_9_enabled }}
           MATRIX_LINUX_5_10_ENABLED: ${{ inputs.linux_5_10_enabled }}
           MATRIX_LINUX_6_0_ENABLED: ${{ inputs.linux_6_0_enabled }}

.github/workflows/cxx_interop.yml

@@ -73,6 +73,11 @@ on:
         description: "Environment variables for Windows jobs as JSON (e.g., '{\"DEBUG\":\"1\",\"LOG_LEVEL\":\"info\"}')."
         default: "{}"
 
+      minimum_swift_version:
+        type: string
+        description: "Minimum Swift version to include in the matrix. Leave empty for auto-detection from Package.swift, specify a version like '6.0' to override, or set to 'none' to disable filtering."
+        default: ""
+
 jobs:
   construct-matrix:
     name: Construct Cxx interop matrix
@@ -104,6 +109,7 @@ jobs:
         env:
           MATRIX_LINUX_COMMAND: "curl -s --retry 3 https://raw.githubusercontent.com/apple/swift-nio/main/scripts/check-cxx-interop-compatibility.sh | bash"
           MATRIX_LINUX_SETUP_COMMAND: "swift --version && apt-get update -y -q && apt-get install -y -q curl jq"
+          MATRIX_MIN_SWIFT_VERSION: ${{ inputs.minimum_swift_version }}
           MATRIX_LINUX_5_9_ENABLED: ${{ inputs.linux_5_9_enabled }}
           MATRIX_LINUX_5_10_ENABLED: ${{ inputs.linux_5_10_enabled }}
           MATRIX_LINUX_6_0_ENABLED: ${{ inputs.linux_6_0_enabled }}

.github/workflows/release_builds.yml

@@ -105,6 +105,11 @@ on:
         description: "Environment variables for Windows jobs as JSON (e.g., '{\"DEBUG\":\"1\",\"LOG_LEVEL\":\"info\"}')."
         default: "{}"
 
+      minimum_swift_version:
+        type: string
+        description: "Minimum Swift version to include in the matrix. Leave empty for auto-detection from Package.swift, specify a version like '6.0' to override, or set to 'none' to disable filtering."
+        default: ""
+
 jobs:
   construct-matrix:
     name: Construct release build matrix
@@ -136,6 +141,7 @@ jobs:
         env:
           MATRIX_LINUX_SETUP_COMMAND: "swift --version"
           MATRIX_LINUX_COMMAND: "swift build -c release"
+          MATRIX_MIN_SWIFT_VERSION: ${{ inputs.minimum_swift_version }}
           MATRIX_LINUX_5_10_ENABLED: ${{ inputs.linux_5_10_enabled }}
           MATRIX_LINUX_5_10_COMMAND_ARGUMENTS: ${{ inputs.linux_5_10_arguments_override }}
           MATRIX_LINUX_6_0_ENABLED: ${{ inputs.linux_6_0_enabled }}

.github/workflows/unit_tests.yml

@@ -145,6 +145,11 @@ on:
         description: "Environment variables for Windows jobs as JSON (e.g., '{\"DEBUG\":\"1\",\"LOG_LEVEL\":\"info\"}')."
         default: "{}"
 
+      minimum_swift_version:
+        type: string
+        description: "Minimum Swift version to include in the matrix. Leave empty for auto-detection from Package.swift, specify a version like '6.0' to override, or set to 'none' to disable filtering."
+        default: ""
+
 jobs:
   construct-matrix:
     name: Construct unit test matrix
@@ -178,6 +183,7 @@ jobs:
         env:
           MATRIX_LINUX_SETUP_COMMAND: "swift --version"
           MATRIX_LINUX_COMMAND: "swift test"
+          MATRIX_MIN_SWIFT_VERSION: ${{ inputs.minimum_swift_version }}
           MATRIX_LINUX_5_9_ENABLED: ${{ inputs.linux_5_9_enabled }}
           MATRIX_LINUX_5_9_COMMAND_ARGUMENTS: ${{ inputs.linux_5_9_arguments_override }}
           MATRIX_LINUX_5_10_ENABLED: ${{ inputs.linux_5_10_enabled }}