Prepare async index removal of token for ci_builds

What does this MR do and why?

As titled, prepare the async index removal of index_ci_builds_on_token_encrypted for ci_builds.

Because index_ci_builds_on_token_encrypted will be replaced with unique_ci_builds_token_encrypted_and_partition_id to help with the partition for ci_builds

How to set up and validate locally

1. Verify the index does exist

   gitlabhq_development=# \di+ index_ci_builds_on_token_encrypted;
                                      List of relations
   Schema |             Name             | Type  |  Owner   |   Table   | Size  | Description
   --------+------------------------------+-------+----------+-----------+-------+-------------
   public | index_ci_builds_on_token_encrypted | index | joe_1565 | ci_builds | 255 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_on_token_encrypted;
   Did not find any relation named "index_ci_builds_on_token_encrypted".

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.

Related to #391957 (closed)