Update net-im/mtxclient and net-im/nheko to latest upstream releases

These are paired bits of software: nheko is the only consumer of
mtxclient in FreeBSD ports. The release notes for nheko are the
interesting (end-user-visible) bits, at
https://github.com/Nheko-Reborn/nheko/releases/tag/v0.8.0
https://github.com/Nheko-Reborn/mtxclient/releases/tag/v0.4.0
with the following highlights:

• Voice and Video Calls (not enabled in ports, needs gstreamer)
• Cross-Signing and Device/User Verification
• Separate profiles

Various ports-tidy things have happened as well:

• Drop googletest build requirement
• Drop libsodium
• Sort dependencies
• Tidy plist a bit