CI: limit the on push triggers (#35574)

Since now we have workflow_dispatch, we can trigger a workflow manually in case we don't have an open PR.

Also, remove the custom `ci skip` code; it's supported natively for some time now: https://github.blog/changelog/2021-02-08-github-actions-skip-pull-request-and-push-workflows-with-skip-ci/

.github/workflows/browserstack.yml

@@ -2,6 +2,11 @@ name: BrowserStack
 
 on:
   push:
+    branches:
+      - main
+  pull_request:
+    branches:
+      - main
   workflow_dispatch:
 
 env:
@@ -14,7 +19,7 @@ permissions:
 jobs:
   browserstack:
     runs-on: ubuntu-latest
-    if: github.repository == 'twbs/bootstrap' && (!contains(github.event.commits[0].message, '[ci skip]') && !contains(github.event.commits[0].message, '[skip ci]'))
+    if: github.repository == 'twbs/bootstrap'
     timeout-minutes: 30
 
     steps: