chore(ci): Add CI for checking minimal versions

Catch if we have under-specified any semver dependency

.github/workflows/ci.yml

@@ -23,7 +23,7 @@ jobs:
     permissions:
       contents: none
     name: CI
-    needs: [test, msrv, lockfile, docs, rustfmt, clippy]
+    needs: [test, msrv, lockfile, docs, rustfmt, clippy, minimal-versions]
     runs-on: ubuntu-latest
     if: "always()"
     steps:
@@ -72,6 +72,27 @@ jobs:
       run: cargo hack check --all-features --locked --rust-version --ignore-private --workspace --all-targets
     - name: No-default features
       run: cargo hack check --no-default-features --locked --rust-version --ignore-private --workspace --all-targets
+  # Make sure the library builds with all dependencies downgraded to their
+  # oldest versions allowed by the semver spec. This ensures we have not
+  # under-specified any dependency
+  minimal-versions:
+    name: Minimal versions
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout repository
+      uses: actions/checkout@v4
+    - name: Install stable Rust
+      uses: dtolnay/rust-toolchain@stable
+      with:
+        toolchain: stable
+    - name: Install nightly Rust
+      uses: dtolnay/rust-toolchain@stable
+      with:
+        toolchain: nightly
+    - name: Downgrade dependencies to minimal versions
+      run: cargo +nightly generate-lockfile -Z minimal-versions
+    - name: Compile with minimal versions
+      run: cargo +stable check --workspace --all-features --all-targets --locked
   lockfile:
     runs-on: ubuntu-latest
     steps: