GSteamer1: Uppdate to 1.28.5

multimedia/gstreamer1-plugins-rust is now a first-class citizen using gstreamer
tagging.

Refactor custom targets in multimedia/gstreamer1 and fix a few bugs in
generate-vuxml-entry.py.

MFH: 2026Q3
Security: 6eee35e6-fca5-4db2-ab73-f27add3e3ef1