Infer rubocop ruby version for guides from the gemspec as well

This requires a rubocop version bump to correctly work. See rubocop/rubocop#13293

Followup to rails#53062 / rails#53137

Author: Earlopain

Gemfile.lock

@@ -477,18 +477,17 @@ GEM
     retriable (3.1.2)
     rexml (3.2.6)
     rouge (4.2.0)
-    rubocop (1.64.0)
+    rubocop (1.67.0)
       json (~> 2.3)
       language_server-protocol (>= 3.17.0)
       parallel (~> 1.10)
       parser (>= 3.3.0.2)
       rainbow (>= 2.2.2, < 4.0)
-      regexp_parser (>= 1.8, < 3.0)
-      rexml (>= 3.2.5, < 4.0)
-      rubocop-ast (>= 1.31.1, < 2.0)
+      regexp_parser (>= 2.4, < 3.0)
+      rubocop-ast (>= 1.32.2, < 2.0)
       ruby-progressbar (~> 1.7)
       unicode-display_width (>= 2.4.0, < 3.0)
-    rubocop-ast (1.31.3)
+    rubocop-ast (1.32.3)
       parser (>= 3.3.1.0)
     rubocop-md (1.2.2)
       rubocop (>= 1.0)

guides/.rubocop.yml

@@ -1,9 +1,6 @@
 inherit_from:
   - '../.rubocop.yml'
 
-AllCops:
-  TargetRubyVersion: 3.0
-
 Style/StringLiterals:
   Enabled: false
 

guides/rails_guides/generator.rb

@@ -1,6 +1,5 @@
 # frozen_string_literal: true
 
-require "set"
 require "fileutils"
 require "nokogiri"
 require "securerandom"