16.0 Advanced Search: Remove old Advanced Search migrations

Background

For every major release, we deprecate old Advanced Search migrations. This is the issue for deprecating migrations in the next major release %16.0.

Proposal

Follow the instructions for removing old Advanced Search migrations.

https://docs.gitlab.com/ee/development/search/advanced_search_migration_styleguide.html#deleting-advanced-search-migrations-in-a-major-version-upgrade

Release Notes

This is a breaking change that enables easier Advanced Search migrations for upgrading GitLab.

Indexes that have not finished migrations up to %15.11 will no longer be supported after GitLab %16.0

Customers will need complete all Advanced Search migrations before upgrading to the latest major version https://docs.gitlab.com/ee/development/search/advanced_search_migration_styleguide.html#deleting-advanced-search-migrations-in-a-major-version-upgrade