[main][Automation] Update versions (#5273)

These files are used for picking agent versions in 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: 8.16.0

```json
{
  "UpgradeToVersion": "8.16.0",
  "CurrentMajors": 1,
  "PreviousMajors": 1,
  "PreviousMinors": 2,
  "SnapshotBranches": [
    "8.15",
    "8.14",
    "7.17"
  ]
}
```

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Denis <denis.rechkunov@elastic.co>

Author: github-actions[bot]

.agent-versions.json

@@ -1,9 +1,9 @@
 {
   "testVersions": [
+    "8.15.0",
     "8.15.0-SNAPSHOT",
     "8.14.4-SNAPSHOT",
     "8.14.3",
-    "8.13.4",
     "7.17.24-SNAPSHOT",
     "7.17.23"
   ]