[META] Stable release 2.x

This is a roadmap meta issue for the next stable 2.x release.

I have been going through the issue queue and it looks like many issues duplicate each other (at least partially).
This is because the module wasn't ready or designed to work with the previous core version.

Here is the issue tracking by problem type.

Reported problems:

1. Modifying field group layout:
   • #3392514: Changing field group layout causes error - changing layout
   • #3389431: Sub-groups can be moved where they don't belong - changing layout + moving regions
2. PHP errors/warnings:
   • #3416584: Error after update to drupal 10.2.2 - can't reproduce on latest stable Drupal version (10.2.7) and using POC of field_group_layout 2.x
   • [✅] #3394083: Failed to open stream - fixed by setting correct path to theme.inc
   • #3433241: Adding field groups causes errors - mix of all the above issues

Additional problems found:

1. css classes applied incorrectly
2. layout styles not loaded

Milestones

1. [🟢] Do not allow regions to be moved outside of a parent group.
2. [🟢] Allow changing layout.
3. [🟢] Ensure that css classes work correctly and styles load correctly.

Legend:

• 🟢 - implemented, needs testing
• 🔴 - not implemented
• ✅ - fixed, tested
• 🕒 - in progress

Notes:
Be aware that if you install the layout_builder (LB) module and change the field group layout to use one of the layouts provided by LB and then uninstall it, you'll get WSOD.
To fix this you must either go to each layout group (/admin/structure/types/manage/[mycontenttype]/display/[group_test]/delete) and delete it,
or you can reinstall the LB module and change the field group layout to use a different one.