Switch to Calendar Versioning

Now that we have multiple openEO backend instances and variants, it's getting more important but harder to figure out what (feature/bugfix) is running where. More actively bumping version numbers helps a lot here as it can be inspected directly in capabilities doc.

We're currently at:
https://github.com/Open-EO/openeo-python-driver/blob/0e1d4a72504448cdfc499688ab875995cf684f48/openeo_driver/_version.py#L1

However, we'll probably never decide to stamp a 1.0.0 version, as we're working in eternal iteration mode, so we're wasting part of our versioning gamut. As a result, the minor part (now at 79) will grow unboundedly, and we're giving off an eternal beta vibe, while we're running production stuff already for a long time.

A popular solution for this problem is https://calver.org/ where you, for example, use the current year as "major" component, and reset "minor" and "patch" level at the beginning of each year. I think this versioning scheme signals is a more honest reflection of the state of this library.