Run CI on any PR change (#14086)

This changes the V4 CI to run on any pull request change (so an opened,
reopened, and updated PR), regardless if the PR is directed into the
`next` branch or not.

This is helpful for testing stacked PRs like:
#14078

Author: philipp-spiess

.github/workflows/ci.yml

@@ -4,7 +4,6 @@ on:
   push:
     branches: [master, next]
   pull_request:
-    branches: [master, next]
 
 permissions:
   contents: read