Ensure no leftover RuboCop version checks

As we may increase the minimum RuboCop version requirement soon, we
should make sure that if we do, we don't leave behind redundant version
checks.

This adds a test to enforce they are cleaned up.

Author: sambostock

Diff for: test/config_test.rb

@@ -90,6 +90,32 @@ def test_no_cops_are_configured_as_pending
     ERROR_MESSAGE
   end
 
+  def test_config_does_not_check_for_rubocop_versions_below_minimum_version
+    rubocop_version_requirement = Gem
+      .loaded_specs.fetch('rubocop-shopify')
+      .dependencies.find { _1.name == 'rubocop' }
+      .requirement
+
+    # RuboCop version checks are done in ERB, so we need to read the raw file
+    redundant_rubocop_version_checks = File.read('rubocop.yml').each_line.with_index.filter_map do |line, index|
+      match = line.match(/<% if rubocop_version >= "(?<version>.*)" %>/)
+      next unless match
+
+      minimum_version_for_config = Gem::Version.new(match[1])
+      next if rubocop_version_requirement.satisfied_by?(minimum_version_for_config)
+
+      [index, line.chomp]
+    end
+
+    assert(redundant_rubocop_version_checks.empty?, <<~ERROR_MESSAGE.chomp)
+      The following RuboCop version check(s) are redundant given the gemspec's version requirement (#{rubocop_version_requirement}):
+
+      #{redundant_rubocop_version_checks.map { "    #{_1.to_s.rjust(4)}: #{_2}" }.join("\n")}
+
+      Please remove these check(s), or (if you intend to maintain compatibility) loosen the `rubocop` requirement in the gemspec.
+    ERROR_MESSAGE
+  end
+
   private
 
   def checking_rubocop_version_compatibility?