Use ActiveRecord::Tasks::DatabaseTasks.migrations_paths explicit for db tasks

[rngtng]

This defines source of truth for migrations_paths for running db rake tasks clearly by passing ActiveRecord::Tasks::DatabaseTasks.migrations_paths explicit to Migrator and avoids setting Migrator.migrations_paths instead.

As its mainly a cosmetic change I couldn't think of any way write a test. Ideas?

Follow up on #21527 /cc @senny

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[senny]

I kept the assignment of ActiveRecord::Migrator.migrations_paths for now (a29db73...dfb89c9). While not immediately obvious this has some side effects on code further down the stack. I'm working on a refactoring towards removing it entirely.

@rngtng thank you 💛