[main][Automation] Update .agent-versions.json (elastic#4602)

This file is used for picking agent versions in integration tests.

The file's 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:

```json
{
  "UpgradeToVersion": "8.15.0",
  "CurrentMajors": 1,
  "PreviousMajors": 1,
  "PreviousMinors": 2,
  "SnapshotBranches": [
    "8.14",
    "8.13",
    "7.17"
  ]
}
```

Co-authored-by: obltmachine <[email protected]>

.agent-versions.json

@@ -2,7 +2,6 @@
   "testVersions": [
     "8.13.3-SNAPSHOT",
     "8.13.2",
-    "8.12.3-SNAPSHOT",
     "8.12.2",
     "7.17.21-SNAPSHOT",
     "7.17.20"