[8.18][Automation] Update versions (#9446)

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": "8.18.6-SNAPSHOT",
  "build_id": "8.18.6-fb45f13c",
  "manifest_url": "https://snapshots.elastic.co/8.18.6-fb45f13c/manifest-8.18.6-SNAPSHOT.json",
  "summary_url": "https://snapshots.elastic.co/8.18.6-fb45f13c/summary-8.18.6-SNAPSHOT.html",
  "core_version": "8.18.6",
  "stack_build_id": "8.18.6-fb45f13c-SNAPSHOT"
}
```

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

```json
{
  "UpgradeToVersion": "8.18.6",
  "CurrentMajors": 1,
  "PreviousMajors": 1,
  "PreviousMinors": 2,
  "SnapshotBranches": [
    "9.1",
    "9.0",
    "7.17"
  ]
}
```

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

Author: github-actions[bot]

.package-version

@@ -1,8 +1,8 @@
 {
   "version": "8.18.6-SNAPSHOT",
-  "build_id": "8.18.6-a6dec873",
-  "manifest_url": "https://snapshots.elastic.co/8.18.6-a6dec873/manifest-8.18.6-SNAPSHOT.json",
-  "summary_url": "https://snapshots.elastic.co/8.18.6-a6dec873/summary-8.18.6-SNAPSHOT.html",
+  "build_id": "8.18.6-fb45f13c",
+  "manifest_url": "https://snapshots.elastic.co/8.18.6-fb45f13c/manifest-8.18.6-SNAPSHOT.json",
+  "summary_url": "https://snapshots.elastic.co/8.18.6-fb45f13c/summary-8.18.6-SNAPSHOT.html",
   "core_version": "8.18.6",
-  "stack_build_id": "8.18.6-a6dec873-SNAPSHOT"
+  "stack_build_id": "8.18.6-fb45f13c-SNAPSHOT"
 }