Resolve "Enable btree_gin postgresql extension"

What does this MR do?

Enable the btree_gin extension in PostgreSQL.

The current index on namespaces.traversal_ids is a gin index to maximize read query performance. A discussion of gin vs gist can be found at https://www.postgresql.org/docs/12/textsearch-indexes.html

Please also note this concern from @stanhu

Related issues

There is a corresponding documentation MR to complement this MR.

The btree_gin extension is needed to create an index of traversal_ids, id pairs. This index is needed to reduce the cost of group - project joins.

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened

Closes #6223 (closed)