Tests: revert concurrency group change

- It's common for us to merge to main and cherry pick to 3.x-stable,
  so it's best if concurrency is shared between branches, which
  is effectively what we had already as it matches on workflow name
  and browser. Ideally, it could also match on the corresponding commit,
  but it seems the commit message is not available in the github context.

Close gh-5492

.github/workflows/browserstack.yml

@@ -15,7 +15,7 @@ jobs:
       NODE_VERSION: 20.x
     name: ${{ matrix.BROWSER }}
     concurrency:
-      group: ${{ matrix.BROWSER }}-${{ github.ref }}
+      group: ${{ github.workflow }}-${{ matrix.BROWSER }}
     timeout-minutes: 30
     strategy:
       fail-fast: false