[Fix rubocop#10871] Restore RuboCop::ConfigLoader.project_root as d…

…eprecated

Fixes rubocop#10871 and utkarsh2102/rubocop-packaging#44.

This PR restores `RuboCop::ConfigLoader.project_root` as deprecated.

It fixes the following build error.

```console
% cd path/to/github.com/utkarsh2102/rubocop-packaging
% bundle update && bundle exec rake
(snip)

Failures:

  1) RuboCop::Cop::Packaging::BundlerSetupInTests when `require
  bundler/setup` is used in a Rakefile does not register an offense
     Failure/Error: let(:project_root) {
  RuboCop::ConfigLoader.project_root }

     NoMethodError:
       undefined method `project_root' for RuboCop::ConfigLoader:Class
     # ./spec/rubocop/cop/packaging/bundler_setup_in_tests_spec.rb:6:in
       `block (2 levels) in <top (required)>'
     # ./spec/rubocop/cop/packaging/bundler_setup_in_tests_spec.rb:39:in
       `block (3 levels) in <top (required)>'
     # ./spec/rubocop/cop/packaging/bundler_setup_in_tests_spec.rb:43:in
       `block (3 levels) in <top (required)>'
```

I considered a soft deprecation, but decided to issue a warning as it
is probably of limited use.

changelog/fix_restore_rubocop_config_loader_project_root.md

@@ -0,0 +1 @@
+* [#10871](https://github.com/rubocop/rubocop/issues/10871): Restore `RuboCop::ConfigLoader.project_root` as deprecated. ([@koic][])

lib/rubocop/config_loader.rb

@@ -137,6 +137,18 @@ def default_configuration
         end
       end
 
+      # Returns the path RuboCop inferred as the root of the project. No file
+      # searches will go past this directory.
+      # @deprecated Use `RuboCop::ConfigFinder.project_root` instead.
+      def project_root
+        warn Rainbow(<<~WARNING).yellow
+          `RuboCop::ConfigLoader.project_root` is deprecated and will be removed in RuboCop 2.0. \
+          Use `RuboCop::ConfigFinder.project_root` instead.
+        WARNING
+
+        ConfigFinder.project_root
+      end
+
       PENDING_BANNER = <<~BANNER
         The following cops were added to RuboCop, but are not configured. Please set Enabled to either `true` or `false` in your `.rubocop.yml` file.