Properly enhance multi-db partitioning rake tasks

What does this MR do and why?

Related to #352770 (closed)

With the addition of multiple databases, many of the rake tasks now have variants for each database, like:

rails db:migrate
rails db:migrate:main
rails db:migrate:ci

This MR fixes existing use of enhance to create dynamic partitions on all variants of these tasks.

How to set up and validate locally

1. Setup multiple databases according https://docs.gitlab.com/ee/development/database/multiple_databases.html#development-setup
2. Drop the existing database (or you can configure a new one if you need data in yours, as long as the schema is not loaded)

   rails db:drop && rails db:create

3. Load the schema in the main database only:

   rails db:structure:load:main

   or with:

   rails db:schema:load:main

4. Look for partitions on a partitioned table, like audit_events. On master the partitions will not exist, but on this branch they will:

   -- master
   \d audit_events
   ...
   Number of partitions: 0
   -- this branch
   \d audit_events
   ...
   Number of partitions: 8

MR acceptance checklist

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