Add to rubocop rule preventing index additions to tables currently on Step 2

We are currently in the process of migrating primary keys for the following tables

• taggings
• ci_builds_metadata
• events

Additional index creation on these tables could cause us to have to do more work during the migrations (example !66625 (comment 659665947)). There is an existing rubocop rule that we can modify to prevent developers from adding indexes to these tables while we work on them.

• Original issue: #332886 (closed)
• Code https://gitlab.com/gitlab-org/gitlab/-/blob/master/rubocop/cop/migration/prevent_index_creation.rb#L11