net-im/mtxclient: update to latest upstream

Release notes at
https://github.com/Nheko-Reborn/mtxclient/releases

Since we skipped 0.5.0: supports edits, API changes, TLS support,
sticker packs and emotes and all that rich-IM-drip. This isn't
compatible with the nheko version in ports, though, waiting for
an update there.