Unpin minitest version to 5.21

Author: aidanharan

Gemfile

@@ -11,7 +11,7 @@ gem "pg",      ">= 0.18.0"
 gem "sqlite3", "~> 1.4"
 gem "tzinfo-data", platforms: [:mingw, :mswin, :x64_mingw, :jruby]
 gem "benchmark-ips"
-gem "minitest", ">= 5.15.0", "< 5.22.0"
+gem "minitest", ">= 5.15.0"
 gem "msgpack", ">= 1.7.0"
 
 if ENV["RAILS_SOURCE"]