Tweaks for the ruby/spec workflow

.github/workflows/spec_guards.yml

@@ -1,4 +1,4 @@
-name: Rubyspec Guard Checks
+name: Rubyspec Version Guards Check
 
 on: [push, pull_request]
 
@@ -9,11 +9,9 @@ jobs:
     if: "!contains(github.event.head_commit.message, '[ci skip]')"
     strategy:
       matrix:
+        # Specs from ruby/spec should still run on all supported Ruby versions.
+        # This also ensures the needed ruby_version_is guards are there, see spec/README.md.
         ruby:
-#         - ruby-2.1
-#         - ruby-2.2
-#         - ruby-2.3
-#         - ruby-2.4
           - ruby-2.5
           - ruby-2.6
           - ruby-2.7
@@ -23,6 +21,7 @@ jobs:
       - uses: ruby/setup-ruby@v1
         with:
           ruby-version: ${{ matrix.ruby }}
+          bundler: none
       - run: ruby ../mspec/bin/mspec
         working-directory: spec/ruby
       - uses: k0kubun/action-slack@v2.0.0