[main][Automation] Update versions (#6007)

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: 9.0.0

```json
{
  "UpgradeToVersion": "9.0.0",
  "CurrentMajors": 1,
  "PreviousMajors": 1,
  "PreviousMinors": 2,
  "SnapshotBranches": [
    "8.x",
    "8.16",
    "7.17"
  ]
}
```

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

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

@@ -6,7 +6,7 @@
 
 testVersions:
   - 8.17.0-SNAPSHOT
+  - 8.16.0
   - 8.16.0-SNAPSHOT
-  - 8.15.3
-  - 8.15.2
+  - 8.15.4
   - 7.17.26-SNAPSHOT