Fix an error when using changed_parameters by external library

This PR fixes the following error when using `changed_parameters` in obsoletion.yml
by external library.

```console
NoMethodError: undefined method `merge' for
[{"cops"=>["Layout/SpaceAroundOperators", "Style/SpaceAroundOperators"],
"parameters"=>"MultiSpaceAllowedForOperators", "reason"=>"If your
intention was to allow extra spaces for alignment, please use `AllowFoy

          first.merge(second)
               ^^^^^^
/Users/koic/src/github.com/rubocop/rubocop-rails/lib/rubocop/rails/inject.rb:11:in `new'
/Users/koic/src/github.com/rubocop/rubocop-rails/lib/rubocop/rails/inject.rb:11:in `defaults!'
/Users/koic/src/github.com/rubocop/rubocop-rails/lib/rubocop-rails.rb:13:in `<top (required)>'
```

changelog/fix_an_error_when_using_changed_parameters_by_external_library.md

@@ -0,0 +1 @@
+* [#10831](https://github.com/rubocop/rubocop/pull/10831): Fix an error when using `changed_parameters` in obsoletion.yml by external library. ([@koic][])

lib/rubocop/config_obsoletion.rb

@@ -47,10 +47,15 @@ def reject_obsolete!
 
     # Default rules for obsoletions are in config/obsoletion.yml
     # Additional rules files can be added with `RuboCop::ConfigObsoletion.files << filename`
-    def load_rules
+    def load_rules # rubocop:disable Metrics/AbcSize
       rules = self.class.files.each_with_object({}) do |filename, hash|
         hash.merge!(YAML.safe_load(File.read(filename))) do |_key, first, second|
-          first.merge(second)
+          case first
+          when Hash
+            first.merge(second)
+          when Array
+            first.concat(second)
+          end
         end
       end
 

spec/rubocop/config_obsoletion_spec.rb

@@ -573,5 +573,28 @@
         expect { config_obsoletion.reject_obsolete! }.not_to raise_error
       end
     end
+
+    context 'when using `changed_parameters` by an external library' do
+      after { described_class.files = [described_class::DEFAULT_RULES_FILE] }
+
+      let(:hash) { {} }
+      let(:external_obsoletions) do
+        create_file('external/obsoletions.yml', <<~YAML)
+          changed_parameters:
+            - cops: Rails/FindEach
+              parameters: IgnoredMethods
+              alternatives:
+                - AllowedMethods
+                - AllowedPatterns
+              severity: warning
+        YAML
+      end
+
+      it 'allows the extracted cops' do
+        described_class.files << external_obsoletions
+
+        expect { config_obsoletion.reject_obsolete! }.not_to raise_error
+      end
+    end
   end
 end