change(consensus): Lower mandatory checkpoint height

[arya2]

Motivation

We want to support full validation of blocks from Canopy activation onwards, particularly for full validation of Canopy blocks on Regtest (though this use case also relies on supporting block construction for Canopy blocks).

Closes #8430.

Specifications & References

https://zips.z.cash/zip-0212

Solution

Lowers mandatory checkpoint height to the block before Canopy activation.

Tests

The mandatory checkpoint height test was updated to expect the last pre-Canopy block height.

Running a sync test locally with checkpoint sync disabled and an added checkpoint at height 1_046_399 until it gets past the grace period

Follow-up Work

PR Author's Checklist

• The PR name will make sense to users.
• The PR provides a CHANGELOG summary.
• The solution is tested.
• The documentation is up to date.
• The PR has a priority label.

PR Reviewer's Checklist

• The PR Author's checklist is complete.
• The PR resolves the issue.

[arya2]

The sync test with checkpoint sync disabled passed the end of the grace period.