x11-wm/river: take maintainership and update to 0.4.2
Notable changes:
- Move MASTER_SITES to correct upstream https://codeberg.org/river/river
- Switch to USES=zig for fetching, building, and installing
- Move pkg-plist to dedicated file
- Remove stale post-patch target
- Change description and COMMENT to reflect new goal of the project
Approved by: lwhsu (mentor, implicitly)
Differential Revision: https://reviews.freebsd.org/D55906