Port to libsoup 3

After #353 (closed) we should port to libsoup 3

• Update dependencies (set libsoup 3/remove libsoup 2 build options in Flatpak manifest)
• Change librest to the API-breaking 1.0 supporting libsoup 3
• Update GIR import (in main.js) to specify version 3
• Adapt the JS code using libsoup in Maps (if any further changes needed)