main/postmarketos-ui-plasma-mobile: switch to modemmanager

Upstream has switched from ofono to modemmanager since Plasma 5.23 and Plasma Mobile Gear 21.12. Since the latter has just been released it's now time to switch our images too.

The modemmanager service will be enabled to start on default and for existing installations the .post-upgrade script will disable and stop the existing ofono service and enable and start modemmanager instead.

The switch does require a proper reboot to start the new process /usr/lib/libexec/modem-daemon, or we need to figure out a way to start it as the user on package upgrade (which I don't think we can do).