It’s expected that it’s “oldstable”, as Debian released Trixie which is now the new “stable”, in Debian’s nomenclature, therefore Bookworm is “oldstable”
YunoHost is not compatible yet with Trixie, it’s currently in publc beta, cf 🎃 YunoHost 13.0 / Trixie spooky beta
Honestly it’s not a big deal to keep being on Bookworm a few months until Trixie is stable-enough for your taste