Drupal 8.9.x deprecation signposting

Problem/Motivation

I've just committed #3096648: Add support for third party libraries in site specific and install profile specific libraries folders to 9.0.x and 8.9.x, this meant that libraries module should not be necessary in Drupal 9. It's not backported to 8.8.x though since it's a new feature.

Sites with libraries module installed will need to uninstall it as part of preparations for a 9.0.x update. I see three ways to do this:

1. Update to 8.9.x, uninstall libraries, then update to 9.0.x
2. Uninstall libraries immediately prior to the 9.0.x update

Then there is option #3 which requires help from libraries module

3. Libraries provides a new version of the module, compatible with 9.0.x but only including a .info.yml file. This allows sites to update to 9.0.x first, then uninstall libraries at any time after the update.

Proposed resolution

Document the options on the project page, possibly add the stub 9.0.x compatible release.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet