Refresh K8up CLI greeting and copyright notice

[tobru]

Context

The greeting of the CLI is boilerplatey, let's have a proper greeting there.

Current greeting:

k8up - A new cli application

Also the copyright notice is wrong.

Implementation Ideas

Change the greeting into:

k8up - Kubernetes Backup Operator

And also update the copyright notice to match the website:

(c) k8up Authors 2021-2023

Make the date dynamic.