Switch to actions/labeler@v4 for labeling PRs. (#14203)
This has a number of advantages: - It allows us to make PR labeling event-driven instead of polling. This, in turn, means that it only runs when a PR is modified, and only evaluates that specific PR, which results in spending far less time running PR labeler jobs. - This is the ‘official’ way to handle PR labels automatically as code, and thus Ilya will no longer need to maintain a custom labeling action for this purpose. - The matching syntax supported by this action is more expressive than what is supported by our existing infrastructure, allowing for things sucha s requring all of a group of files to be modified in a PR for it to have a specific label applied. We have no need right now for this, but it is useful to have it available long-term. - Because the labeling is PR-triggered, it shows up in the PR checks, so we can more easily see if auto-labeling has run for a given PR or not.