Add an option to delete the local built-in group "admin"

The NC setup creates a user "admin" and a group "admin". While the admin user can be deleted, the admin group cannot. Moreover, it seems that there are certain permissions checks which are hard-coded for the literal string admin, see bug #42480. On top there is already an option to delegate some administrative privileges to other groups via ./settings/admin/admindelegation.

I would like to see an option which

1. allows to delete the built-in group admin
2. grant another group full admin rights

The feature administration delegation already provides a foundation for aspect 2. All what is missing is a new "super"-capability which grants full admin powers or (alternatively) additional capabilities which fill out the missing parts. This would also make the LDAP feature unnecessary which allows to promote a single LDAP group as admin group via php occ ldap:promote-group. Instead of a special feature which only works for LDAP-based groups (and which currently seems to be partially broken, see #42480), this would provide a generic solution to the problem.

Start by reading the administration delegation feature at ./settings/admin/admindelegation and the existing php occ ldap:promote-group behavior. Trace how the built-in admin group and delegated administrative capabilities are enforced, including the permissions checks referenced by bug #42480. Done means the built-in group can be deleted and another group can receive complete administrative rights without LDAP-specific handling.