CI: update to ruby:3.3 to fix build

Site build is currently broken.

Upgrade to ruby:3.3 to fix it.

This appears to work:

• https://gitlab.freedesktop.org/pvir/pipewire-org/-/jobs/61016228
• https://pvir.pages.freedesktop.org/pipewire-org/ (it's looking for stylesheet in /style.css so in my private pages it doesn't find it, should work OK on pipewire.org)