Give GroupScimAuthAccessToken its own serializer

Series

group_id has been dead since group SCIM tokens moved to group_scim_auth_access_tokens in 17.9.

Prerequisites — target master, any order, all must land before step 2:

MR	What
!261148 👈	GroupScimAuthAccessToken gets its own serializer
!261149	Real group_id only on the group SCIM API entity
!261150 (merged)	No SCIM token transfer on group org transfer

Steps — sequential, one milestone apart, since each needs the previous deployed, not just merged:

Step	MR	Milestone	What	State
1	!256720 (merged)	19.5	Delete orphaned group-level rows	merged
2	!256721	19.6	Remove code references, add ignore_column	open
3	!256722	19.7	Drop group_id column	open
4	!256725	19.8	Remove ignore_column rule	open

What does this MR do and why?

ScimOauthAccessToken and GroupScimAuthAccessToken shared one entity that branched on group to pick the SCIM URL. This gives the group model its own entity and drops the branch.

No behaviour change — the group model always has a group, so it always took that branch. Prerequisite for !256721, where ScimOauthAccessToken stops responding to group.

References

• #624518

How to set up and validate locally

No behavior change, as the instance SCIM auth tokens have no group ID. A green pipeline should be enough.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist.