Backward compatibility to work `Marshal.load(legacy_record.dump)` for MySQL

Context https://github.com/rails/rails/pull/39599#discussion_r445429365.

Similar with #39611.

Actually we never said we would keep Mashal compatibility between two
versions of Rails, so we sometimes directly removed the dedicated types
in the past (e.g. f1a0fa9e, #29666), but that direct removing makes cache
rotation harder.

Keeping the unused constant in new version at least during one version
will help cache rotation easier.

activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb

@@ -844,6 +844,10 @@ def version_string(full_version_string)
           full_version_string.match(/^(?:5\.5\.5-)?(\d+\.\d+\.\d+)/)[1]
         end
 
+        # Alias MysqlString to work Mashal.load(File.read("legacy_record.dump")).
+        # TODO: Remove the constant alias once Rails 6.1 has released.
+        MysqlString = Type::String # :nodoc:
+
         ActiveRecord::Type.register(:immutable_string, adapter: :mysql2) do |_, **args|
           Type::ImmutableString.new(true: "1", false: "0", **args)
         end

activerecord/test/cases/base_test.rb

@@ -1226,6 +1226,21 @@ def test_marshalling_with_associations
     assert_equal 1, post.comments.length
   end
 
+  if current_adapter?(:Mysql2Adapter)
+    def test_marshal_load_legacy_6_0_record_mysql
+      path = File.expand_path(
+        "support/marshal_compatibility_fixtures/legacy_6_0_record_mysql.dump",
+        TEST_ROOT
+      )
+      topic = Marshal.load(File.read(path))
+
+      assert_not_predicate topic, :new_record?
+      assert_equal 1, topic.id
+      assert_equal "The First Topic", topic.title
+      assert_equal "Have a nice day", topic.content
+    end
+  end
+
   if Process.respond_to?(:fork) && !in_memory_db?
     def test_marshal_between_processes
       # Define a new model to ensure there are no caches