Use bootc service units for updates

[castrojo]

Updates for bootc are handled by:

bootc-fetch-apply-updates.service
bootc-fetch-apply-updates.timer

Note that by default it will do an --apply, which is a reboot! So we should probably ship an override.conf.

[bsherman]

I'm not sure if we should do that in this repo.

My goal for CCOS is to be a more stable, mostly vanilla upstream alternative to FCOS.

But given SCOS is a thing, CCOS is already not really vanilla upstream, so maybe there is room for some tweaks, I just don't want to keep the scope tight.