chore(db): Drop unsed index on last_seen column in GroupRelease table

[vgrozdanic]

Similar as in #91610, index on last_seen column hasn't been used since Oct 29th (since then we track the usage), and it takes 674GiB of disk in our main US DB.

I went through our codebase and couldn't find any query that would still be using this index. On our main US DB (and some other single tenants) this will be executed by an SRE, since dropping the index will likely timeout, even as a post deployment migration. For other regions, this index is small enough to be removed as a post deployment migration

[github-actions]

This PR has a migration; here is the generated SQL for src/sentry/migrations/0890_remove_index_on_group_release_last_seen.py

for 0890_remove_index_on_group_release_last_seen in sentry

--
-- Alter field last_seen on grouprelease
--
DROP INDEX CONCURRENTLY IF EXISTS "sentry_grouprelease_last_seen_aff155f8";