Simplify versioning

The versioning here was originally inherited from the Fedora CoreOS
configuration. However...the version numbering was always
overridden by coreos-assembler, so it wasn't actually used there!

Conceptually there are two things here:

- OS version
- Arbitrary date stamp

For the "OS version"...well, the closest thing we have actually
to "version of set of RPMs" is a compose today, which is expressed
in a distinct label already - at least for CentOS and RHEL.
For Fedora of course post-branching there are no "composes"
as such but just a set of floating RPMs post-release.

We have the "arbitrary date stamp" in the container image build
time already - and tooling like bootc and rpm-ostree show
both the version and the build time.

Let's significantly simplify our version numbers by just going
to "OS version".

This especially fixes the bug that we weren't setting
`releasever` anymore which just broke the version anyways.

Closes: https://gitlab.com/fedora/bootc/base-images/-/issues/40

Signed-off-by: Colin Walters <walters@verbum.org>

minimal/postprocess-conf.yaml

@@ -25,6 +25,3 @@ check-passwd:
 check-groups:
   type: "file"
   filename: "group"
-
-automatic-version-prefix: "${releasever}.<date:%Y%m%d>"
-mutate-os-release: "${releasever}"