generate_release_dates.py: add "Define Release" and "Review Priorities"

[pdurbin]

What this PR does / why we need it:

Which issue(s) this PR closes:

• Closes add additional dates to release date script #12445

Special notes for your reviewer:

Suggestions on how to test this:

Run the script with dates from milestones at https://github.com/IQSS/dataverse/milestones

An example:

% python3 scripts/dev/release-dates/generate_release_dates.py 2026-12-16
- Define Release: 2026-09-10
- Start: Sprint ??, 2026-09-24
- Review Priorities: 2026-11-12
- Community PR Last Call: 2026-11-19
- Core PR Last Call: 2026-11-26
- Code Freeze: 2026-12-03
- Curation Team Review: 2026-12-07 (allocate 5 full days)
- Release Date: 2026-12-16

Make sure the new fields are present and that the dates make sense.

Does this PR introduce a user interface change? If mockups are available, please link/include them here:

Is there a release notes update needed for this change?:

Additional documentation:

[github-actions]

Test Results

405 tests   386 ✅  28m 6s ⏱️
 55 suites   15 💤
 55 files      4 ❌

For more details on these failures, see this check.

Results for commit 9858e29.