Fix: Workflow trigger updated to run only on PRs to main (#7087)

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.

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

Author: rlazo

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

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