chore: set nv25 calibnet upgrade epoch

[rjan90]

Related Issues

Timing confirmed by implementers: https://filecoinproject.slack.com/archives/C05P37R9KQD/p1741938432377989

Proposed Changes

Set nv25 calibnet upgrade epoch to 2520574, which corresponds to:

• Los Angeles: 2025-03-25T16:00:00Z
• London time: 2025-03-25T23:00:00Z
• UTC: 2025-03-25T23:00:00Z
• Bejing time: 2025-03-26T07:00:00Z

Additional Info

Checklist

Before you mark the PR ready for review, please make sure that:

• Commits have a clear commit message.
• PR title conforms with contribution conventions
• Update CHANGELOG.md or signal that this change does not need it per contribution conventions
• New features have usage guidelines and / or documentation updates in
  • Lotus Documentation
  • Discussion Tutorials
• Tests exist for new functionality or change in behavior
• CI is green