Update a user's user_group_member_roles when group membership is updated

What does this MR do and why?

Update a user's user_group_member_roles when either member_role_id or access_level value of their group membership is updated.

References

Implements part of Create user_group_member_roles when group membe... (#515215 - closed).

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Start GDK in SAAS mode

   $ GITLAB_SIMULATE_SAAS=1 gdk start

2. Enable feature flag

   $ rails c
   > Feature.enable(:cache_user_group_member_roles)

3. Login with root, go to a group that root owns (e.g. http://localhost:9393/groups/flightjs), then create two new member roles in http://localhost:9393/groups/flightjs/-/settings/roles_and_permissions page
4. Add a new member to the group and assign them the first member role
5. In Rails console, verify that a new Authz::UserGroupMemberRole is created for the invited user

   $ rails c
   > Authz::UserGroupMemberRole.last
   => #<Authz::UserGroupMemberRole
     ...     
     member_role_id: <id_of_first_member_role>,
     >

6. Update the membership of the user: change the member role to the other member role
7. In Rails console, verify that the Authz::UserGroupMemberRole record of the user is updated: member_role_id is now that of the second member role

   $ rails c
   > Authz::UserGroupMemberRole.last
   => #<Authz::UserGroupMemberRole
     ...     
     member_role_id: <id_of_second_member_role>,
     >

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