multimedia/plexmediaserver: update to 1.31.3.6868

Changelog: https://forums.plex.tv/t/plex-media-server/30447/568

NEW:
(MacOS) This is the last release to support MacOS 10.11-10.12
(Windows) This is the last release to support Windows 7-8.1

FIXES:
(CreditsDetection) Improve temporary directory clean-up.
(NativeSeriesScanner) Addressed an issue where certain

filenames could falsely register as an episode range.

(Scanner) Certain episode filenames could lead to excessive

memory usage.

(Transcoder) Converting certain files could be unreliable

when using Nvidia GPUs with certain driver versions.

(Windows) 64-bit installer would not install on Windows ARM64.

PR: 270500
Reported by: Ben Shertenlieb <b.shertenlieb@yahoo.com> (maintainer)