[main][Automation] Update .agent-versions.json (#4535)

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": "8.14.0",
  "CurrentMajors": 2,
  "PreviousMajors": 1,
  "PreviousMinors": 1,
  "SnapshotBranches": [
    "8.13",
    "8.12",
    "7.17"
  ]
}
```

* Test 8.12.2 instead of 8.13.0
* Change version requirements to match the manual selection

---------

Co-authored-by: apmmachine <apmmachine@users.noreply.github.com>
Co-authored-by: Craig MacKenzie <craig.mackenzie@elastic.co>
Co-authored-by: Denis Rechkunov <denis.rechkunov@elastic.co>

Author: 3 people

.agent-versions.json

@@ -1,10 +1,10 @@
 {
   "testVersions": [
-    "8.13.0-SNAPSHOT",
+    "8.13.2-SNAPSHOT",
+    "8.13.1",
     "8.12.3-SNAPSHOT",
     "8.12.2",
-    "8.12.1",
-    "7.17.19-SNAPSHOT",
-    "7.17.18"
+    "7.17.20-SNAPSHOT",
+    "7.17.19"
   ]
 }

magefile.go

@@ -1606,8 +1606,8 @@ func (Integration) UpdateVersions(ctx context.Context) error {
 
 	reqs := upgradetest.VersionRequirements{
 		UpgradeToVersion: bversion.Agent,
-		CurrentMajors:    2,
-		PreviousMinors:   1,
+		CurrentMajors:    1,
+		PreviousMinors:   2,
 		PreviousMajors:   1,
 		SnapshotBranches: branches,
 	}