Squash core migrations (#2842)

Data migrations (seed default groups, seed default permissions) are deliberately excluded.
This also allows us to remove a lot of now unnecessary public API from the migrator and migration repository.

src/Console/ConsoleServiceProvider.php

@@ -46,6 +46,8 @@ class ConsoleServiceProvider extends AbstractServiceProvider
                 ResetCommand::class,
                 ScheduleListCommand::class,
                 ScheduleRunCommand::class
+                // Used internally to create DB dumps before major releases.
+                // \Flarum\Database\Console\GenerateDumpCommand::class
             ];
         });