assumeutxo: Drop block height from metadata

Summary:
Removing the block height from the assumeutxo snapshot file prevents potential UB (implicit-integer-sign-change) when loading the file. The snapshot file should be considered untrusted input, and its fields should be sanitized or removed if unnecessary.

The Snapshot format version is updated to 2 to indicate this change.

Co-authored-by: MarcoFalke <*~=`'#}+{/-|&$^_@721217.xyz>

This is a backport of core#30598
Depends on D18059

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18060