Assign release milestone fails on pull requests: the token lacks pull-requests: write

The Assign release milestone workflow added in #252 fails on every merged pull request: the step that sets the milestone of the PR gets 403 Resource not accessible by integration (runs on the merges of #252 and #256). Its issue side works (#255 got the 26.11.1 milestone). Setting a milestone on a pull request needs pull-requests: write; the workflow has pull-requests: read.