CI: Test with oldest allowed version of RuboCop

.github/workflows/main.yml

@@ -70,6 +70,26 @@ jobs:
       - name: Use latest RuboCop from `master`
         run: |
           echo "gem 'rubocop', github: 'rubocop/rubocop'" > Gemfile.local
+          cat Gemfile.local
+      - uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: "3.3"
+          bundler-cache: true
+      - run: NO_COVERAGE=true bundle exec rake ${{ matrix.task }}
+
+  oldest-rubocop:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        task:
+          - spec
+    name: "Oldest RuboCop: ${{ matrix.task }}"
+    steps:
+      - uses: actions/checkout@v4
+      - name: Use oldest RuboCop allowed by gemspec
+        run: |
+          sed -nr "s/spec.add_dependency 'rubocop', '~> ([0-9\.]+)'/gem 'rubocop', '= \1'/p" rubocop-rspec.gemspec > Gemfile.local
+          cat Gemfile.local
       - uses: ruby/setup-ruby@v1
         with:
           ruby-version: "3.3"

rubocop-rspec.gemspec

@@ -37,5 +37,5 @@ Gem::Specification.new do |spec|
     'rubygems_mfa_required' => 'true'
   }
 
-  spec.add_runtime_dependency 'rubocop', '~> 1.40'
+  spec.add_dependency 'rubocop', '~> 1.40'
 end