Tighten the RuboCop AST dependency (#9158)

Users can upgrade upcoming RuboCop safely, even if incompatible APIs
are introduced in future RuboCop AST major updates.

The purpose is to reduce the runtime impact for users, as it is already
specified `rubocop-ast` in the Gemfile so that development can try the latest.

rubocop.gemspec

@@ -35,7 +35,7 @@ Gem::Specification.new do |s|
   s.add_runtime_dependency('rainbow', '>= 2.2.2', '< 4.0')
   s.add_runtime_dependency('regexp_parser', '>= 1.8', '< 3.0')
   s.add_runtime_dependency('rexml')
-  s.add_runtime_dependency('rubocop-ast', '>= 1.2.0')
+  s.add_runtime_dependency('rubocop-ast', '>= 1.2.0', '< 2.0')
   s.add_runtime_dependency('ruby-progressbar', '~> 1.7')
   s.add_runtime_dependency('unicode-display_width', '>= 1.4.0', '< 2.0')