[9.1][Automation] Update versions

These files are used for picking the starting (pre-upgrade) or ending (post-upgrade) agent versions in upgrade integration tests.

The content is based on responses from https://www.elastic.co/api/product_versions and https://snapshots.elastic.co

The current update is generated based on the following requirements:

`.package-version`

```json
{
  "version": "9.1.5-SNAPSHOT",
  "build_id": "9.1.5-5028ec55",
  "manifest_url": "https://snapshots.elastic.co/9.1.5-5028ec55/manifest-9.1.5-SNAPSHOT.json",
  "summary_url": "https://snapshots.elastic.co/9.1.5-5028ec55/summary-9.1.5-SNAPSHOT.html",
  "core_version": "9.1.5",
  "stack_build_id": "9.1.5-5028ec55-SNAPSHOT"
}
```

`testing/integration/testdata/.upgrade-test-agent-versions.yml`

```json
{
  "UpgradeToVersion": "9.1.5",
  "CurrentMajors": 1,
  "PreviousMajors": 1,
  "PreviousMinors": 2,
  "SnapshotBranches": [
    "9.1",
    "9.0",
    "8.19",
    "7.17"
  ]
}
```

Author: github-actions[bot]

.package-version

@@ -1,8 +1,8 @@
 {
-  "version": "9.1.4-SNAPSHOT",
-  "build_id": "9.1.4-01a3f106",
-  "manifest_url": "https://snapshots.elastic.co/9.1.4-01a3f106/manifest-9.1.4-SNAPSHOT.json",
-  "summary_url": "https://snapshots.elastic.co/9.1.4-01a3f106/summary-9.1.4-SNAPSHOT.html",
-  "core_version": "9.1.4",
-  "stack_build_id": "9.1.4-01a3f106-SNAPSHOT"
+  "version": "9.1.5-SNAPSHOT",
+  "build_id": "9.1.5-5028ec55",
+  "manifest_url": "https://snapshots.elastic.co/9.1.5-5028ec55/manifest-9.1.5-SNAPSHOT.json",
+  "summary_url": "https://snapshots.elastic.co/9.1.5-5028ec55/summary-9.1.5-SNAPSHOT.html",
+  "core_version": "9.1.5",
+  "stack_build_id": "9.1.5-5028ec55-SNAPSHOT"
 }

testing/integration/testdata/.upgrade-test-agent-versions.yml

@@ -5,8 +5,8 @@
 # upgrade integration tests.
 
 testVersions:
+  - 9.0.8-SNAPSHOT
   - 9.0.7
-  - 9.0.7-SNAPSHOT
+  - 8.19.5-SNAPSHOT
   - 8.19.4
-  - 8.19.4-SNAPSHOT
   - 7.17.30-SNAPSHOT