Add schema version check

See https://gitlab.com/gitlab-org/gitlab/-/issues/207458

config/initializers/fill_shards.rb

-# The `table_exists?` check is needed because during our migration rollback testing,
+# The explicit schema version check is needed because during our migration rollback testing,
 # `Shard.connected?` could be cached and return true even though the table doesn't exist
 return unless Shard.connected?
-return unless Shard.table_exists?
-return unless Shard.connection.index_exists?(:shards, :name, unique: true)
+return unless ActiveRecord::Migrator.current_version >= 20190402150158
 return if Gitlab::Database.read_only?
 
 Shard.populate!