Improve GitHub Actions hygiene for Dagger

See google/go-github#1821

This CL makes a few changes to reduce the load of GitHub Actions from Dagger:
  * Restricts GitHub Actions to only the master or gh-pages branches
  * Cancels previous GitHub Action runs for the same PR branch.
  * Removes excessive API levels for emulator tests.

RELNOTES=N/A
PiperOrigin-RevId: 362394043

.github/workflows/docs-ci.yml

@@ -5,6 +5,8 @@ on:
     branches:
       - gh-pages
   pull_request:
+    branches:
+      - gh-pages
 
 defaults:
   run: