Allow loading of Hashie::Mash in YAML

We have some legacy database entries in `ci_trigger_request.variables`
that serialized values as `Hashie::Mash`. This results in a 500 error
when attempting to load old jobs. Allow this class to be deserialized
since it's effectively a `Hash`.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/440316

Changelog: fixed