DEV: Promote old post-deploy migrations to pre-deploy migrations (#13477)

Having a large number of post-deploy migrations running out-of-numerical-sequence with pre-deploy migrations can be problematic. For example, if we have the sequence

- db/migrate/2017... - add column
- db/post_migrate/2018... - drop the column
- db/migrate/2021... - add the same column again

It will work fine in numerical order. But if you run the pre-deploy migrations **followed by** the post-deploy migrations, you will not get the same result.

Our post-deploy system is designed to allow for seamless upgrades of Discourse. However, it is reasonable for us to only support this totally seamless experience for a limited period of time. This commit moves all post_deploy migrations which are more than 1 year old (i.e. more than 2 major Discourse versions ago) into the regular pre-deploy migrations directory. This limits the impact of any edge cases caused by out-of-numerical-sequence migrations.

lib/migration/safe_migrate.rb

@@ -6,7 +6,6 @@ class Discourse::InvalidMigration < StandardError; end
 
 class Migration::SafeMigrate
   module SafeMigration
-    UNSAFE_VERSION = 20180321015220
     @@enable_safe = true
 
     def self.enable_safe!
@@ -19,7 +18,7 @@ class Migration::SafeMigrate
 
     def migrate(direction)
       if direction == :up &&
-         version && version > UNSAFE_VERSION &&
+         version && version > Migration::SafeMigrate.earliest_post_deploy_version &&
          @@enable_safe != false &&
          !is_post_deploy_migration?
 
@@ -153,4 +152,12 @@ class Migration::SafeMigrate
       raise Discourse::InvalidMigration, "Attempt was made to rename or delete column"
     end
   end
+
+  def self.earliest_post_deploy_version
+    @@earliest_post_deploy_version ||= begin
+      first_file = Dir.glob("#{Discourse::DB_POST_MIGRATE_PATH}/*.rb").sort.first
+      file_name = File.basename(first_file, ".rb")
+      file_name.first(14).to_i
+    end
+  end
 end