refactor: Use versioning in tags consistent with mkosi's internal $IMAGE_VERSION

[Smujb]

Switch to an image naming scheme similar to Universal Blue, where images are named YYYYMMDD.V instead of simply YYYYMMDD. The V is a version number for disambiguation and it increments by 1 for every build on the same day.

mkosi.bump has been rewritten in Python to first check the existing mkosi.version and increment it if necessary according to this versioning scheme. Index starts at 0, so the first image built on any given day will be YYYYMMDD.0. This phases out the messy hack that was used before of using YYYYMMDDHHMMSS as the version mkosi.bump would return and then separately versioning the image itself.

Features:

• /etc/os-release now has the version of the currently booted image under IMAGE_VERSION
• bootc status will list the version associated with every deployment
• All of this versioning is consistent with the tags on Github, so users know exactly which image they are on
• Every build that succeeds, CI or local, will have a new version ID. This will be the case even when multiple images are built on the same day.

Note: This versioning scheme requires images to already follow it in order to work. So the first image built after merging this will end in .0 even if it was not the first image built on that day.

fixes #54