Fixes #36651 - Support Minitest 5.19+

• This reverts commit 1c3a4155f286352e0abbf0d0b298e47e81c2d6c5.
  Thus un-pinning minitest again
• update minitest-spec-rails to 7.1+
  This contains a fix required for minitest 5.19+
• Update mocha to 2.1+ for minitest 5.19+ compat
• Use minitest (5) support in shoulda-matchers
  We've not been using Minitest 4 for a while now
• Use Minitest namespace, not MiniTest
• Don't override metadata, minitest doesn't like that