Reduce overhead of LabelFinder by avoiding #presence call

Some users experienced 502 timeouts when viewing group labels.
Labels#open_issues_count and Label#open_merge_requests_count were
taking a long time to load because they were loading every ActiveRecord
of the user-accessible projects into memory. This change modifies so
that only the IDs are loaded into memory.

Closes #23684

See merge request !7094
Signed-off-by: Rémy Coutable <remy@rymai.me>