[8.x][Automation] Update versions (#6896)

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: 8.19.0

```json
{
  "UpgradeToVersion": "8.19.0",
  "CurrentMajors": 1,
  "PreviousMajors": 1,
  "PreviousMinors": 2,
  "SnapshotBranches": [
    "9.0",
    "8.x",
    "7.17"
  ]
}
```

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

Author: github-actions[bot]

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

@@ -5,9 +5,8 @@
 # upgrade integration tests.
 
 testVersions:
-  - 8.18.0-SNAPSHOT
-  - 8.17.1-SNAPSHOT
-  - 8.17.0
-  - 8.16.2
+  - 8.19.0-SNAPSHOT
+  - 8.17.2
+  - 8.16.4
   - 7.17.28-SNAPSHOT
   - 7.17.27