Add release checklist

While not an officially supported file, in my experience it is helpful to have a release checklist available. This makes the release process more transparent and less dependent on specific maintainers.

The checklist documents the release steps and can be updated via PRs to add additional steps/adjust existing steps whenever needed.

The checklist should be used as the PR description for the release PR from `develop` to `main`.

.github/release-checklist.md

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+# Template to use for release PRs from `develop` to `main`
+
+:warning: **DO NOT MERGE (YET)** :warning:
+
+**Please **do** add approvals if you agree as otherwise we won't be able to release.**
+
+PR for tracking changes for the x.x.x release. Target release date: **DOW MONTH DAY YEAR**.
+
+## Release checklist
+
+### General
+
+- [ ] Verify, and if necessary, update the allowed version ranges for various dependencies in the `composer.json` - PR #xxx
+- [ ] PHPCS: check if there have been releases since the last WordPressCS release and check through the changelog to see if there is anything WordPressCS could take advantage of - PR #xxx
+- [ ] PHPCSUtils: check if there have been releases since the last WordPressCS release and update WordPressCS code to take advantage of any new utilities - PR #xxx
+- [ ] PHPCSExtra: check if there have been releases since the last WordPressCS release and check through the changelog to see if there is anything WordPressCS could take advantage of - PR #xxx
+- [ ] Check if the minimum WP version property needs updating in `MinimumWPVersionTrait::$default_minimum_wp_version` and if so, action it - PR #xxx
+- [ ] Check if any of the list based sniffs need updating and if so, action it.
+    List based sniffs:
+    - [ ] `WordPress.WP.ClassNameCase` - PR #xxx
+    - [ ] `WordPress.WP.DeprecatedClasses` - PR #xxx
+    - [ ] `WordPress.WP.DeprecatedFunctions` - PR #xxx
+    - [ ] `WordPress.WP.DeprecatedParameters` - PR #xxx
+    - [ ] `WordPress.WP.DeprecatedParameterValues` - PR #xxx
+- [ ] Check if any of the other lists containing information about WP Core need updating and if so, action it.
+    - [ ] `$allowed_core_constants` in `WordPress.NamingConventions.PrefixAllGlobals` - PR #xxx
+    - [ ] `$pluggable_functions` in `WordPress.NamingConventions.PrefixAllGlobals` - PR #xxx
+    - [ ] `$pluggable_classes` in `WordPress.NamingConventions.PrefixAllGlobals` - PR #xxx
+    - [ ] `$target_functions` in `WordPress.Security.PluginMenuSlug` - PR #xxx
+    - [ ] `$reserved_names` in `WordPress.NamingConventions.ValidPostTypeSlug` - PR #xxx
+    - [ ] `$wp_time_constants` in `WordPress.WP.CronInterval` - PR #xxx
+    - [ ] `$known_test_classes` in `IsUnitTestTrait` - PR #xxx
+    - [ ] ...etc...
+
+### Release prep
+
+- [ ] Add changelog for the release - PR #xxx
+    :pencil2: Remember to add a release link at the bottom!
+- [ ] Update `README` (if applicable) - PR #xxx.
+- [ ] Update wiki (new customizable properties etc) (if applicable)
+
+### Release
+
+- [ ] Merge this PR
+- [ ] Make sure all CI builds are green.
+- [ ] Tag and create a release against `main` (careful, GH defaults to `develop`!) & copy & paste the changelog to it.
+    :pencil2: Check if anything from the link collection at the bottom of the changelog needs to be copied in!
+- [ ] Make sure all CI builds are green.
+- [ ] Close the milestone
+- [ ] Open a new milestone for the next release
+- [ ] If any open PRs/issues which were milestoned for this release did not make it into the release, update their milestone.
+- [ ] Fast-forward `develop` to be equal to `main`
+
+### After release
+
+- [ ] Open a Trac ticket for WordPress Core to update.
+
+### Publicize
+
+- [ ] [Major releases only] Publish post about the release on Make WordPress
+- [ ] Tweet, toot, etc about the release.
+- [ ] Post about it in Slack.
+- [ ] Submit for "Month in WordPress": https://make.wordpress.org/community/month-in-wordpress-submissions/