Fix failed ee/spec/requests/api/dora/metrics_spec.rb

What does this MR do?

Fixes master:broken.

Dora::AggregateMetricsService takes metrics within last 3 months by default: https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/services/dora/aggregate_metrics_service.rb#L87

The created metrics in tests don't fall into that range anymore. Let's create date dynamically in order to fix the tests.