Draft: [SPIKE] Execute migrations ordered by milestone

DO NOT MERGE!

What does this MR do and why?

This is a PoC (#416223 (closed)), to find out what we need to achieve Migrations should run in milestone, then type, ... (&10411 - closed).

We have the following migrations:

db/migrate/20230713043152_test_migration_1.rb
db/migrate/20230713043226_test_migration_3.rb
db/migrate/20230713043244_test_migration_5.rb
db/migrate/20230713043256_test_migration_6.rb
db/post_migrate/20230713043135_test_migration_4.rb
db/post_migrate/20230713043309_test_migration_2.rb
db/post_migrate/20230713043322_test_migration_7.rb

With the changes from this MR, the migrations are executed as following:

1. Migrations with no milestone specified are executed first, ordered by their version (timestamp)
2. Migrations with milestone specified are executed as:
   1. First, compare milestones
   2. If milestones are the same, we first execute regular migrations, then post-deployment migrations.
   3. For migrations with the same milestone, and of the same type, we sort them by version (timestamp), as what Rails does by default.

So the above migrations end up executed as this:

$ bundle exec rails db:migrate:main
main: == 20230713043152 TestMigration1: migrating ===================================
main: == 20230713043152 TestMigration1: migrated (0.0048s) ==========================

main: == 20230713043309 TestMigration2: migrating ===================================
main: == 20230713043309 TestMigration2: migrated (0.0023s) ==========================

main: == 20230713043226 TestMigration3: migrating ===================================
main: == 20230713043226 TestMigration3: migrated (0.0021s) ==========================

main: == 20230713043135 TestMigration4: migrating ===================================
main: == 20230713043135 TestMigration4: migrated (0.0019s) ==========================

main: == 20230713043244 TestMigration5: migrating ===================================
main: == 20230713043244 TestMigration5: migrated (0.0020s) ==========================

main: == 20230713043256 TestMigration6: migrating ===================================
main: == 20230713043256 TestMigration6: migrated (0.0026s) ==========================

main: == 20230713043322 TestMigration7: migrating ===================================
main: == 20230713043322 TestMigration7: migrated (0.0020s) ==========================

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