Schedule external_key index removal

What does this MR do and why?

Schedule external_key index removal

Schedules the removal of the external_key index on the issues table.

Changelog: removed

References

• Part of Delete external_key column (&18754 - closed)

Index usage: https://dashboards.gitlab.net/goto/Nv0CULwNg?orgId=1

Screenshots or screen recordings

Before	After

How to set up and validate locally

 bin/rails db:migrate
main: == [advisory_lock_connection] object_id: 144600, pg_backend_pid: 71399
main: == 20250801143759 RemoveIndexIssuesOnProjectIdAndExternalKey: migrating =======
main: -- index_exists?(:issues, [:project_id, :external_key], {:name=>"index_issues_on_project_id_and_external_key"})
main:    -> 0.0224s
main: -- quote_column_name("index_issues_on_project_id_and_external_key")
main:    -> 0.0000s
main: == 20250801143759 RemoveIndexIssuesOnProjectIdAndExternalKey: migrated (0.0524s)

main: == [advisory_lock_connection] object_id: 144600, pg_backend_pid: 71399

select * from postgres_async_indexes order by id desc limit 1;
 id |          created_at           |          updated_at           |                    name                     |                              definition                               | table_name | attempts | last_error
----+-------------------------------+-------------------------------+---------------------------------------------+-----------------------------------------------------------------------+------------+----------+------------
 38 | 2025-08-01 15:08:12.570166+00 | 2025-08-01 15:08:12.570166+00 | index_issues_on_project_id_and_external_key | DROP INDEX CONCURRENTLY "index_issues_on_project_id_and_external_key" | issues     |        0 |
(1 row)

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