Suppress RuboCop offenses

This commit suppresses the following new RuboCop offenses:

```console
$ bundle exec rubocop
(snip)

rubocop-minitest.gemspec:35:8: C: [Correctable] Gemspec/AddRuntimeDependency:
Use add_dependency instead of add_runtime_dependency.
  spec.add_runtime_dependency 'rubocop', '>= 1.61', '< 2.0'
       ^^^^^^^^^^^^^^^^^^^^^^
rubocop-minitest.gemspec:36:8: C: [Correctable] Gemspec/AddRuntimeDependency:
Use add_dependency instead of add_runtime_dependency.
  spec.add_runtime_dependency 'rubocop-ast', '>= 1.31.1', '< 2.0'
       ^^^^^^^^^^^^^^^^^^^^^^

136 files inspected, 2 offenses detected, 2 offenses autocorrectable
```

rubocop-minitest.gemspec

@@ -32,6 +32,6 @@ Gem::Specification.new do |spec|
   spec.executables = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
   spec.require_paths = ['lib']
 
-  spec.add_runtime_dependency 'rubocop', '>= 1.61', '< 2.0'
-  spec.add_runtime_dependency 'rubocop-ast', '>= 1.31.1', '< 2.0'
+  spec.add_dependency 'rubocop', '>= 1.61', '< 2.0'
+  spec.add_dependency 'rubocop-ast', '>= 1.31.1', '< 2.0'
 end