rework version handling

This relates to https://gitlab.com/fedora/bootc/base-images/-/tree/wip-baseimage-rework

Previously we were using rpm-ostree compose image with our default config which has mutate-os-release with a timestamp, but now that we're aiming to support moving away from that direct flow due to https://github.com/containers/buildah/issues/5952 - it creates skew because the person building the image is now required to deal with standard labels like org.opencontainers.image.version.

My take is: probably we should just take the hit and stop mutating /usr/lib/os-release to include a version at all. (This also tangentially relates to the topic that if we did mutate, we should use the new IMAGE_VERSION key and not OSTREE_VERSION)

An immediate side effect of this is that there are parts of the stack which only know about os-release and not about the container image metadata (most notably, ostree when writing the bootloader, and systemd).

So from the "inside the OS" perspective (except for bootc) will go back to something like

PRETTY_NAME="Fedora Linux 41"

vs the current

PRETTY_NAME="Fedora Linux 41.20250212.0 (Forty One)"