Fixes iteration spec that relies on dynamic dates

What does this MR do and why?

Fixes iteration spec that relies on dynamic dates

On the user_views_iteration_cadence_spec, an iteration factory is automatically created with dynamic dates (via x.weeks.ago). With the transition to 2025, the dates are displayed to account for the year switch:

• Expectation: Dec 25 - Jan 1, 2025
• Outcome: Dec 25, 2024 - Jan 1, 2025

Failure detail: https://gitlab.com/gitlab-org/gitlab/-/jobs/8799293533.

This commit updates the iteration factory to use fixed dates.

MR acceptance checklist

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• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• [-] The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• [-] This MR has a severity label assigned (if applicable).
• [-] Set the milestone of the merge request to match the target backport branch version.
• This MR has been approved by a maintainer (only one approval is required).
• [-] Ensure the e2e:test-on-omnibus-ee job has either succeeded or been approved by a Software Engineer in Test.

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