Improve release scripts

Opening this to track a few possible improvements to the release scripts.

- [ ] Bring back improvements from the wpjm-addons scripts to core
- [ ] Possibly re-use common parts
- [ ] Look into using [merge queue](https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/configuring-pull-request-merges/managing-a-merge-queue#triggering-merge-group-checks-with-github-actions) to run an action before the release PR is merged, and add the 'Update changelog' commit to the PR
- [ ] Avoid possible overwrite of the PR description when the playground link and plugin build is added
- [ ] Always link to all included PRs in the release PR, to make it easier to go through them. Or maybe to the milestone? This is currently only done for PRs that don't have a valid release notes section
- [ ] Warn about PRs not assigned to a milestone. Add [Type] Maintenance exception
- [ ] Add 'Developer Notes' section based on the Template/hook changes

The issue names no specific files; start by locating the release scripts and comparing them with the wpjm-addons scripts, then trace how release pull requests, changelogs, builds, milestones, and release notes are assembled. Clarify which checklist item is in scope and define completion criteria for that focused change before implementation.

Written by the indexing model from the issue text.