Update bundled gems list at 2021-12-20

NEWS.md

@@ -379,7 +379,7 @@ Note: We're only listing outstanding class updates.
     * minitest 5.15.0
     * power_assert 2.0.1
     * rake 13.0.6
-    * test-unit 3.5.2
+    * test-unit 3.5.3
     * rexml 3.2.5
     * rbs 1.8.1
     * typeprof 0.20.4

gems/bundled_gems

@@ -2,7 +2,7 @@
 minitest 5.15.0 https://github.com/seattlerb/minitest
 power_assert 2.0.1 https://github.com/ruby/power_assert
 rake 13.0.6 https://github.com/ruby/rake
-test-unit 3.5.2 https://github.com/test-unit/test-unit 3.5.2
+test-unit 3.5.3 https://github.com/test-unit/test-unit 3.5.3
 rexml 3.2.5 https://github.com/ruby/rexml
 rss 0.2.9 https://github.com/ruby/rss 0.2.9
 net-ftp 0.1.3 https://github.com/ruby/net-ftp