Pin minitest version to 5.21

Managed to reproduce CI failure at https://buildkite.com/rails/rails-nightly/builds/133#018d7bb8-8a32-4978-8e36-d7cb9b067813/1196-1204 . It would also reproduce against the released versions of Ruby because this is triggered by minitest v5.22.0 change. minitest/minitest@ebb468c

To avoid all of railties CI failures, pin minitest version to 5.21 tentatively.

* Steps to reproduce
```ruby
git clone https://github.com/rails/rails
cd rails
rm Gemfile.lock
bundle install
cd railties
bin/test test/application/test_runner_test.rb -n test_system_tests_are_not_run_with_the_default_test_command
```

* Expected behavior
It should pass.

* Actual behavior
```ruby
$ bin/test test/application/test_runner_test.rb -n test_system_tests_are_not_run_with_the_default_test_command
Run options: -n test_system_tests_are_not_run_with_the_default_test_command --seed 14574

F

Failure:
ApplicationTests::TestRunnerTest#test_system_tests_are_not_run_with_the_default_test_command [test/application/test_runner_test.rb:1191]:
Expected /0\ runs,\ 0\ assertions,\ 0\ failures,\ 0\ errors,\ 0\ skips/ to match "Nothing ran for filter: \nRunning 0 tests in a single process (parallelization threshold is 50)\nRun options: --seed 45713\n\n# Running:\n\n".

bin/test test/application/test_runner_test.rb:1179

Finished in 9.982314s, 0.1002 runs/s, 0.2004 assertions/s.
1 runs, 2 assertions, 1 failures, 0 errors, 0 skips
$
```

Gemfile

@@ -3,7 +3,7 @@
 source "https://rubygems.org"
 gemspec
 
-gem "minitest", ">= 5.15.0"
+gem "minitest", ">= 5.15.0", "< 5.22.0"
 
 # We need a newish Rake since Active Job sets its test tasks' descriptions.
 gem "rake", ">= 13"

Gemfile.lock

@@ -618,7 +618,7 @@ DEPENDENCIES
   libxml-ruby
   listen (~> 3.3)
   mdl (!= 0.13.0)
-  minitest (>= 5.15.0)
+  minitest (>= 5.15.0, < 5.22.0)
   minitest-bisect
   minitest-ci
   minitest-retry