net-im/snac: Update 2.57 → 2.68

There are a large number of improvements and crash fixes between these
versions as documented at Release Notes:
https://codeberg.org/grunfink/snac2/src/tag/2.68/RELEASE_NOTES.md

PR: 284110
Approved by: Matthew Phillips <matthew@matthewphillips.info> (maintainer)
MFH: 2025Q1

(cherry picked from commit ab751c3536153a8ba004f848a91bea943263b03f)