[8.18][Automation] Update versions (#7121)

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: 8.18.0

```json
{
  "UpgradeToVersion": "8.18.0",
  "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>
Co-authored-by: Andrzej Stencel <andrzej.stencel@elastic.co>

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

@@ -7,5 +7,5 @@
 testVersions:
   - 8.17.2
   - 8.16.4
+  - 7.17.29-SNAPSHOT
   - 7.17.28
-  - 7.17.28-SNAPSHOT