Fix issuable search optimization in PG12

Uses a materialized CTE so that our optimization fence actually works.

Recursive CTEs are materialized by default in PG12 but we're not using
this recursively here so we use Gitlab::SQL::CTE which handles adding of
the MATERIALIZED keyword when needed.

Changelog: fixed
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/61880