Mark Layout Builder as a stable module

Problem/Motivation

Take the final steps to mark Layout Builder as a stable module

Proposed resolution

Swap Core (Experimental) for Core in the info file
Remove the @internal tag when only used because the module is experimental (and clarify the tag for the cases it is kept)
Copy the CSS and templates to Stable theme
Add an entry to MAINTAINERS.txt

Remaining tasks

Any remaining "Needs ____" tags, as applicable

Release management checklist

1. Security
2. Data integrity
3. Impact on stable functionality
   

   Integrations

   • QuickEdit
   • Contextual Links
   • Content Moderation
   • Custom Blocks
   • REST
   • JSON:API
4. Maintenance and technical debt
5. BC policy
6. Core gates
   • Usability
   • Accessibility

     Critical accessibility blockers have been completed. Many major issues remain. See #3007978: Accessibility Plan for Layout Builder.

   • Testing
   • Documentation
   • Performance

User interface changes

API changes

Data model changes

Release notes snippet