Fix update-tests-metadata failing in master

We can't use Gitlab::Json here because this is a standalone Ruby script
and we don't load the Rails environment.

We fixed the same problem in https://gitlab.com/gitlab-org/gitlab/-/merge_requests/30937
but we missed another instance of the same problem

lib/rspec_flaky/report.rb

@@ -40,7 +40,7 @@ module RspecFlaky
       report_path_dir = File.dirname(file_path)
       FileUtils.mkdir_p(report_path_dir) unless Dir.exist?(report_path_dir)
 
-      File.write(file_path, Gitlab::Json.pretty_generate(flaky_examples.to_h))
+      File.write(file_path, JSON.pretty_generate(flaky_examples.to_h))
     end
 
     def prune_outdated(days: OUTDATED_DAYS_THRESHOLD)