Draft: Skip migrations from running when `database_tasks: false` is set

What does this MR do and why?

It does use information of database_tasks: false to know that such connections are being shared with main: (a primary connection).

The usage of database_tasks: false makes to not run migrations on top of connection marked as this like ci:. To ensure that database_tasks: true/false is properly used this does implement gitlab:db:validate_config for that.

This does solve migrations skipping/restrict approach since:

1. If database_tasks: true is used, the migration will be run everywhere, but some with restrict* will be skipped (a message will be printed, and they will be marked via schema_migrations as run)
2. If database_tasks: false is used, the migration will be run only on main:, since no db:migrate:ci will exist. Such migration will not be skipped, and instead it will be run in-order on top of main:

This:

• Based on top of !73756 (merged)
• Uses the !82902 (merged) (single commit picked)
• Implements Solution 2 of the #355014 (closed)

This makes it possible to softly skip migrations (print information) if we have two connections using two databases.

Resolves:

• #355014 (closed)

Related:

• #342378 (closed)

MR acceptance checklist

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