Fix broken master by making tests date-independent

What does this MR do and why?

Fix broken master by making tests date-independent!

These were introduced in https://gitlab.com/gitlab-org/gitlab/-/work_items/604039; crucially, we forgot to stub "today" in these tests, so we introduced a time bomb (!).

Fast quarantine at Fast-quarantine secrets management nav badge specs (gitlab-org/quality/engineering-productivity/fast-quarantine!557 - merged) to tide us over until this is merged. See https://gitlab.com/gitlab-org/gitlab/-/work_items/604039#note_3692352275 for links to broken jobs.

Rather than backing out the code now that it's not used, I decided to use this as an opportunity to demonstrate the way we should write these tests in future: stub today's date in the specs to confirm behaviour on both sides of the deadline, leaving nothing to fall back to the ambient time.

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