Fix: Workflow trigger updated to run only on PRs to main

The metalava-semver-check.yml workflow was previously configured to run on all pull requests. This change updates the trigger to only run on pull requests that target the `main` branch.

Author: google-labs-jules[bot]

.github/workflows/metalava-semver-check.yml

@@ -2,6 +2,8 @@ name: Metalava SemVer Check
 
 on:
   pull_request:
+    branches:
+      - main
 
 jobs:
   semver-check: