It was possible to grant access to a group to do things to protected
branches or tags, but if the group were deleted, the deletion
might fail due to a message such as the following:

```
ActiveRecord::InvalidForeignKey: PG::ForeignKeyViolation: ERROR:  update or delete on table "namespaces" violates foreign key constraint "fk_rails_7111b68cdb" on table "protected_branch_push_access_levels"
DETAIL:  Key (id)=(1827859) is still referenced from table "protected_branch_push_access_levels".
: DELETE FROM "namespaces" WHERE "namespaces"."type" IN ('Group') AND "namespaces"."id" = 1827859
```

This MR fixes this by adding missing cascading deletions to the
group_id foreign keys in the following tables:

* protected_branch_merge_access_levels
* protected_branch_push_access_levels
* protected_tag_create_access_levels

Closes #3343, #3344, gitlab-org/gitlab-ce#37311