Install activesupport for verify-tests-metadata

The same idea is used in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/110018

We still use
`Gitlab::RspecFlaky::Report.load(flaky_report_path).flaky_examples.to_h`
because it does the following, which is more than just `JSON.parse`:

* `JSON.parse`
* Check if the parsed JSON is a `Hash`: `collection.is_a?(Hash)` and the
  values can also be converted to be a hash `example.to_h`
* `Time.parse(attributes[attr]) if attributes[attr].is_a?(String)` where
  `attr` is `first_flaky_at` and `last_flaky_at`, and `attributes` is
  `example.to_h`