Prepare Plugin Check 1.9.0 release

[davidperezgar]

Summary

• bump plugin.php version constants and the readme stable tag to 1.9.0
• refresh changelog entries with the v1.9.0 notes from milestone 23
• update docs/checks.md with the new and revised checks

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: davidperezgar <davidperez@git.wordpress.org>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.