Schedule index removal for ci_builds_metadata

Ref: #373239 (closed)

What does this MR do?

This MR schedules an index removal for index_ci_builds_metadata_on_build_id.

Why are we doing this?

Following up !97924 (merged) where we introduced our new index unique index including the partition key.

Indexes:
-   "index_ci_builds_metadata_on_build_id" UNIQUE, btree (build_id)
    "index_ci_builds_metadata_on_build_id_partition_id_unique" UNIQUE, btree (build_id, partition_id)

This MR removes our previous index in order to be able to partition ci_build_metadata with our zero partition helper as all unique constraints must include the partition key.

How to set up and validate locally

1. Verify the index does exist

gitlabhq_development=# \di+ index_ci_builds_metadata_on_build_id;
                                   List of relations
Schema |             Name             | Type  |  Owner   |   Table   | Size  | Description
--------+------------------------------+-------+----------+-----------+-------+-------------
public | index_ci_builds_metadata_on_build_id | index | morefice | security_findings | 76 GB |
(1 row)

2. Enable feature flag

Feature.enable(:database_reindexing)

3. Run migration with bundle exec rails db:migrate
4. Run bundle exec rails gitlab:db:reindex so the index is destroyed asynchronously
5. Verify the index no longer exist

gitlabhq_development=# \di+ index_ci_builds_metadata_on_build_id;
Did not find any relation named "index_ci_builds_metadata_on_build_id".

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.