Fix gin index detection on routes table

backport index detection fix to 14.10 version.

This is a similar change to !87374 (merged) except it is for a different table: routes vs projects. This only needs to be backported to 14.10 stable branch as its corresponding migration !81121 (merged) was introduced in %14.10

• bug issue: #361776 (closed)
• backport issue: https://gitlab.com/gitlab-org/release/tasks/-/issues/3856
• original MR: !87647 (merged)