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.

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