Revert "RuboCop: target the current Ruby version rather than old 2.7 (rails#54692)"

RuboCop does use the current Ruby version by default, but just can't
deduce it itself in certain situations, like in a `test/dummy` app
within a plugin that relies on a `.ruby-version` from the parent
directory.

This reverts commit 7705019.

Author: jeremy

railties/CHANGELOG.md

@@ -1,7 +1,3 @@
-*   RuboCop targets the Ruby version you're using rather than the 2.7 default.
-
-    *Jeremy Daer*
-
 *   Add a `SessionTestHelper` module with `sign_in_as(user)` and `sign_out` test helpers when
     running `rails g authentication`. Simplifies authentication in integration tests.
 

railties/lib/rails/generators/rails/app/templates/rubocop.yml.tt

@@ -1,9 +1,6 @@
 # Omakase Ruby styling for Rails
 inherit_gem: { rubocop-rails-omakase: rubocop.yml }
 
-AllCops:
-  TargetRubyVersion: <%= ENV["RBENV_VERSION"] || ENV["rvm_ruby_string"] || RUBY_ENGINE_VERSION %>
-
 # Overwrite or add rules to create your own house style
 #
 # # Use `[a, [b, c]]` not `[ a, [ b, c ] ]`

railties/lib/rails/generators/rails/plugin/templates/rubocop.yml.tt

@@ -1,9 +1,6 @@
 # Omakase Ruby styling for Rails
 inherit_gem: { rubocop-rails-omakase: rubocop.yml }
 
-AllCops:
-  TargetRubyVersion: <%= ENV["RBENV_VERSION"] || ENV["rvm_ruby_string"] || RUBY_ENGINE_VERSION %>
-
 # Overwrite or add rules to create your own house style
 #
 # # Use `[a, [b, c]]` not `[ a, [ b, c ] ]`

railties/test/generators/app_generator_test.rb

@@ -614,9 +614,6 @@ def test_inclusion_of_rubocop
     run_generator
     assert_gem "rubocop-rails-omakase"
     assert_file "config/environments/development.rb", %r|# Apply autocorrection by RuboCop to files generated by `bin/rails generate`\.|
-    assert_file ".rubocop.yml" do |content|
-      assert_match "TargetRubyVersion: #{ENV["RBENV_VERSION"] || ENV["rvm_ruby_string"] || RUBY_ENGINE_VERSION}", content
-    end
   end
 
   def test_quiet_assets_in_development