Use Gem.ruby_version in test_inclusion_of_ruby_version test

Rails main branch requires Ruby 3.2.0 that ships RubyGems 3.4.1 that is higher than 3.3.3,
so we can use `Gem.ruby_version` in `test_inclusion_of_ruby_version` test.

As per commented at rails#53072 (comment)

Co-authored-by: Earlopain <[email protected]>

Author: yahonda

railties/test/generators/app_generator_test.rb

@@ -1028,10 +1028,8 @@ def test_bootsnap_with_dev_option
   def test_inclusion_of_ruby_version
     run_generator
 
-    ruby_version = "#{Gem::Version.new(Gem::VERSION) >= Gem::Version.new("3.3.13") ? Gem.ruby_version : RUBY_VERSION}"
-
     assert_file "Dockerfile" do |content|
-      assert_match(/ARG RUBY_VERSION=#{ruby_version}/, content)
+      assert_match(/ARG RUBY_VERSION=#{Gem.ruby_version}/, content)
     end
     assert_file ".ruby-version" do |content|
       if ENV["RBENV_VERSION"]