audio/mumble: update to 1.5.517 and take maintainership

Changelog: https://www.mumble.info/blog/mumble-1.5.517-rc/

PR: 261395
Approved by: arrowd (mentor)
Differential Revision: https://reviews.freebsd.org/D41047
MFH: 2023Q3

(cherry picked from commit d055a8e8c80b70e3f826c46e038f5fd83919b290)