sysutils/tmux: Update to 3.6b

This is a bug-fix release, with one reported upstream change to remove
images from the correct list when they are removed while in the
alternate screen.

Changelog: https://raw.githubusercontent.com/tmux/tmux/3.6b/CHANGES
Release issue: https://github.com/tmux/tmux/issues/5109
Sponsored by: The FreeBSD Foundation