x11-toolkits/libXmu: Update to 1.3.1

Update WWW and pkg-descr.

https://lists.x.org/archives/xorg-announce/2024-March/003493.html
https://lists.x.org/archives/xorg-announce/2024-April/003508.html
https://lists.x.org/archives/xorg-announce/2026-January/003655.html
https://lists.x.org/archives/xorg-announce/2026-January/003668.html

PR: 286757
Approved by: x11 (arrowd)
Approved by: fluffy (mentor)