Use HTTP 422 for changelog API errors

In https://gitlab.com/gitlab-org/gitlab/-/merge_requests/53890 it was
brought up that an HTTP 500 error is a very generic error, and that an
HTTP 422 error code is better suited for the changelog API.

This fixes https://gitlab.com/gitlab-org/gitlab/-/issues/321676

lib/api/repositories.rb

@@ -227,7 +227,7 @@ module API
         service.execute
         status(200)
       rescue Gitlab::Changelog::Error => ex
-        render_api_error!("Failed to generate the changelog: #{ex.message}", 500)
+        render_api_error!("Failed to generate the changelog: #{ex.message}", 422)
       end
     end
   end

spec/requests/api/repositories_spec.rb

@@ -687,7 +687,7 @@ RSpec.describe API::Repositories do
         }
       )
 
-      expect(response).to have_gitlab_http_status(:internal_server_error)
+      expect(response).to have_gitlab_http_status(:unprocessable_entity)
       expect(json_response['message']).to eq('Failed to generate the changelog: oops')
     end
   end