Migrate screenshot workflow to playground/cli (Attempt: 2)

[carstingaxion]

Description of the Change

Use WordPress Playground in favor of wp-env as the environment for the screenshot generator.

Switching to Playground not only makes this workflow a lot faster, it also allows to use a blueprint.json to setup the environment, which is way more comfortable, than writing a long list of wp-cli commands to do the same.

Needed before merge

• Finish the docs
• Load requested language by URL WordPress/wordpress-playground#1703 (comment)
• NEW 'Compress Images' workflow #837

Included in this PR is also:

• the subtle change to use the modern Admin-UI,
  which should become the default across all GatherPress Playgrounds
• hiding the admin-notices for this presentation-purpose, like @patriciabt suggested
• fixing the left sided menu background to fill the full height of the screen

Closes #

How to test the Change

Changelog Entry

Changed - Run the screenshot generator workflow on WordPress Playground

Credits

Props @carstingaxion

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.