audio/mumble-server: Update 1.5.857 => 1.5.915

Release Notes:
https://www.mumble.info/blog/mumble-1.5.901/
https://www.mumble.info/blog/mumble-1.5.915/

Changelog:
https://github.com/mumble-voip/mumble/releases/tag/v1.5.901
https://github.com/mumble-voip/mumble/releases/tag/v1.5.915

Sponsored by: UNIS Labs
MFH: 2026Q3