Make `Gitlab::Database::Migration[2.0]` to be default
This MR does the following changes:
- Add documentation for migrations for multiple databases.
- This makes the migrations using a new style being a default (not yet mandatory).
- This updates all exceptions and other documentation places to include link to migration docs.
- Creates
MigrationRecord
as a base class for migrations.
Resolves: #358434 (closed)
Changelog: added
Edited by Kamil Trzciński (Back 2025-01-01)