Cancel GitHub CI workflows which are running on old commits.

This conserves GitHub resources, since the old workflows are no longer relevant.

PiperOrigin-RevId: 523945642

Author: hovinenb

.github/workflows/ci.yml

@@ -6,6 +6,10 @@ on:
   pull_request:
     branches: [ "main" ]
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.ref }}
+  cancel-in-progress: true
+
 permissions: read-all
 
 env: