Ruby 3.0: Use YAML.safe_load_file with permitted_classes

What does this MR do and why?

Otherwise, we are seeing the following exception on Ruby 3.0:

Failure/Error: actual_changes = keep.each_change(&:itself)
 ArgumentError:
   unknown keyword: :permitted_classes

Fixes https://gitlab.com/gitlab-org/gitlab/-/jobs/6215350940

How to set up and validate locally

asdf local ruby 3.0.6
bundle
bin/rspec spec/keeps/delete_old_feature_flags_spec.rb