[policy, no patch] Move Layout Builder module to contrib

Problem/Motivation

This came out of a discussion with @catch, @lauriii, @gabor hojtsy, @pameeela, @xjm, @wim leers, @phenaproxima, @timplunkett, @pdureau, @ckrina (and others who I may have temporarily forgotten who was at which meeting) at DrupalCon Vienna.

Drupal core provides multiple layout tools:

• Theme regions with blocks. See #3555047: Add a way to ship block placement statically (will be useful when block UI is removed)
• Manage display for entity fields. See #3564234: Add display-builder agnostic overview page for display modes where we improved this already.
• Layout Builder

Neither are actively being developed further, they are in maintenance mode mostly in core. The contributed module space is concentrating around building on top of SDC components with Drupal Canvas, Display Builder and other solutions instead to place entity fields, replace/avoid theme regions entirely and sidestep Layout Builder. Neither of those solutions are quite ready yet for core inclusion but given the core layout solutions not being built further this results in a confusing situation for Drupal builders.

Drupal core only used Layout Builder in 2 places: the Umami demo and Navigation's layout.

The UX for editing navigation's layout is by far not up to Drupal standards and is being decoupled in #3521159: Add Navigation Layout module to remove dependency on Layout Builder and then planned to be removed from core. Umami has already been hidden from the Drupal installer as it is not demonstrating Drupal's capabilities representatively anymore. It is now only used for performance testing as a "real scenario". It was built before Layout Builder and already has an MR to restore it to pre-layout builder glory by keeping everything visually exactly intact.

Drupal CMS uses Drupal Canvas extensively and only uses Layout Builder for the Dashboard which is itself also being decoupled.

This matches with the efforts outlined in #3533198: [Meta] Make Drupal the first "design-system native" CMS + Unify & simplify render & theme systems too.

Steps to reproduce

N/A.

Proposed resolution

Deprecate and remove Layout Builder from core.

Remaining tasks

Obtain core leadership signoffs.

User interface changes

Layout builder will not be included in core anymore. It will still be available as a contributed project.

Introduced terminology

None.

API changes

None.

Data model changes

None.

Release notes snippet

None. (This is a policy issue, actual implementation will be separate).