ci: run full matrix on PRs targeting any branch

The pull_request branch filter meant the test/lint/coverage matrix only ran
on PRs targeting main or v1.x. Stacked PRs (targeting feature branches) only
got the conformance checks, which are continue-on-error and don't exercise
unit tests. Removing the filter so the full matrix runs on every PR.

Author: maxisbey

.github/workflows/main.yml

@@ -5,7 +5,6 @@ on:
     branches: ["main", "v1.x"]
     tags: ["v*.*.*"]
   pull_request:
-    branches: ["main", "v1.x"]
 
 permissions:
   contents: read