Make autocomplete prefix sorting case-insensitive

What does this MR do and why?

When sorting matches for autocomplete results, we prioritize matches that start with the search term. This should be case-insensitive to make it consistent with the old frontend sorting behavior when the mention_autocomplete_backend_filtering is disabled.

Before:

SELECT id, username, name
FROM "users"
  INNER JOIN "project_authorizations" ON "users"."id" = "project_authorizations"."user_id"
WHERE "project_authorizations"."project_id" = 278964 AND (REPLACE(users.name, ' ', '') ILIKE '%adam%' OR users.username ILIKE '%adam%')
ORDER BY CASE WHEN starts_with(REPLACE(users.name, ' ', ''), 'adam') OR starts_with(users.username, 'adam') THEN 1 ELSE 2 END, "users"."username" ASC, "users"."id" ASC
LIMIT 10

https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/26870/commands/83691

After:

SELECT id, username, name
FROM "users"
  INNER JOIN "project_authorizations" ON "users"."id" = "project_authorizations"."user_id"
WHERE "project_authorizations"."project_id" = 278964 AND (REPLACE(users.name, ' ', '') ILIKE '%adam%' OR users.username ILIKE '%adam%')
ORDER BY CASE WHEN REPLACE(users.name, ' ', '') ILIKE 'adam%' OR users.username ILIKE 'adam%' THEN 1 ELSE 2 END, "users"."username" ASC, "users"."id" ASC
LIMIT 10

https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/26870/commands/83692

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

1. Enable mention_autocomplete_backend_filtering.
2. Try autocompleting a user and verify that prefix matches are prioritized regardless of case.