[META] Enable layout builder for form displays, and deprecate field_layout

While there is no explicit plan yet, I think this is a more accurate issue title for what we'd like to happen.

LB still doesn't support form modes, so the way remains 📌 Move Field Layout data model and API directly into \Drupal\Core\Entity\EntityDisplayBase Needs work

Should we warn users that the module should probably not be used? I have created 📌 Inform users that Field Layout is deprecated Active .

In today's Usability Meeting 📌 Drupal Usability Meeting 2024-02-09 Needs work we've discussed ✨ Improve the manage form display page by untwining field widgets that belong to the main content and sidebar Active outlining an underlaying problem which is relevant for this issue as well as for the Field Layout contrib module. The issue illustrates that things are already problematic without Field Layout being installed and you only have an enabled and disabled section on the Manage form display page. But with Field Layout set to for example a two column layout the problem gets worse. We've agreed on postponing ✨ Improve the manage form display page by untwining field widgets that belong to the main content and sidebar Active on this issue and bring it on everyone's radar.

@andypost highlighted that this has a product manager review tag. While there is a page building initiative being bootstrapped which will shine a lot more light on layout building that we did in recent years, I doubt that changing how forms are laid out will be a priority of that initiative either anytime soon. So my feeling is this issue will keep lingering here like it did throughout Drupal 9 and 10. If we want a "clean solution" we can deprecate and move it out to contrib like other core modules. Ideally it would be folded into layout builder, but that clearly did not happen in years.