Fix test-unit release date (#51)

Author: hiroyuki-sato

_config.yml

@@ -1,6 +1,6 @@
 markdown: kramdown
 test_unit_version: 3.5.3
-test_unit_release_date: 2022-12-20
+test_unit_release_date: 2021-12-20
 test_unit_rails_version: 6.1.0
 test_unit_rails_release_date: 2021-01-13
 test_unit_activesupport_version: 1.0.9