Update pre_migration_check postgres version requirement (#28800)

lib/tasks/db.rake

@@ -16,8 +16,8 @@ namespace :db do
   end
 
   task pre_migration_check: :environment do
-    version = ActiveRecord::Base.connection.select_one("SELECT current_setting('server_version_num') AS v")['v'].to_i
-    abort 'This version of Mastodon requires PostgreSQL 9.5 or newer. Please update PostgreSQL before updating Mastodon' if version < 90_500
+    version = ActiveRecord::Base.connection.database_version
+    abort 'This version of Mastodon requires PostgreSQL 12.0 or newer. Please update PostgreSQL before updating Mastodon.' if version < 120_000
   end
 
   Rake::Task['db:migrate'].enhance(['db:pre_migration_check'])