Remove deprecated 2020 Learn child theme and parent

[adamwoodnz]

See #2832

• Removes the wporg-learn-2020 theme and the dependency on the wporg parent theme.
• Updates build and lint workflow scripts and configs.
• Removes some now unused code from the plugin.
• Updates the readme.

Note that the wporg-learn theme in SVN is not in this Git repo and needs to be deleted separately.

Testing

1. Make a fresh clone of the repo and check out this branch.
2. Follow the readme and ensure the site starts up successfully with the 2024 theme activated.

To regression test the build you can check out the build-test branch (which was built off this branch), and run it locally, or sync the individual projects to a sandbox.

[adamwoodnz]

Old button styles