Remove unused Grafana compatibility code

[yuri-tceretian]

Description

This pull request removes unused Grafana compatibility code that is no longer relevant and updates related tests accordingly. Additionally, it modifies tests to utilize LoadCompat for improved test consistency.

Changes Made

• Removed obsolete Grafana compatibility code.
• Deleted related tests for the removed code.
• Updated existing tests to use LoadCompat.

Checklist

• Unit tests added/updated
• Documentation updated if necessary
• Code changes reviewed and approved

---

Note

Low Risk
Primarily removes dead code and updates unit tests to use LoadCompat; runtime behavior should be unchanged aside from test fixtures/validation paths.

Overview
Removes unused Alertmanager/Grafana compatibility helpers, including the standalone Load parser and GrafanaToUpstreamConfig converter, and deletes the associated unit test.

Updates alerting config tests to consistently go through LoadCompat and adjusts fixtures to satisfy the stricter receiver validation/defaulting logic (notably fuller EmailConfig fields and switching complex-route fixtures to webhook receivers).

^{Written by Cursor Bugbot for commit fb82038. This will update automatically on new commits. Configure here.}

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{Bugbot Autofix is ON. A Cloud Agent has been kicked off to fix the reported issue.}

[cursor]

Bugbot Autofix prepared fixes for 1 of the 1 bugs found in the latest run.

• ✅ Fixed: Invalid YAML key breaks route test
  • Changed webhook_configs:: (double colon) to webhook_configs: (single colon) on line 352, fixing the invalid YAML syntax that was causing the test to fail during parsing instead of testing route conversion logic.

Or push these changes by commenting:

@cursor push 2c3eb15e14

Preview (2c3eb15e14)

diff --git a/definition/compat_test.go b/definition/compat_test.go
--- a/definition/compat_test.go
+++ b/definition/compat_test.go
@@ -349,6 +349,6 @@
     webhook_configs:
       - url: http://localhost:8080/alert
   - name: recv2
-    webhook_configs::
+    webhook_configs:
       - url: http://localhost:8080/alert
 `