audio/rosegarden: update to 25.12 (latest release) and Qt6

This update skips over three years of updates and switches the build
to use Qt6 instead of Qt5. It adds lv2 support. Upstream release notes
are at
https://www.rosegardenmusic.com/wiki/dev:25.12
but the previous six releases are probably relevant, also.

PR: 289532
Reported-by: diizzy@