Explain how to upgrade k8s version in the docs

Why do you need this?

First of all thanks for k3sup! I can recommend it to everyone! I was looking for a way to upgrade K8s version but couldn't find it in the docs. I reran the bootstrapping commands and that did the trick so it seems to be (at least) possible. A couple of lines on how to upgrade K8s version in the docs would be very helpful.

Expected Behaviour

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Current Behaviour

N.A.

Possible Solution

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Steps to Reproduce

N.A.

Your Environment

• k3sup version:

k3sup version

• What Kubernetes distribution, client and server version are you using?

kubectl version

• What OS or type or VM are you using for your cluster? Where is it hosted? (for k3sup install/join):

• Operating System and version (e.g. Linux, Windows, MacOS):

uname -a

cat /etc/os-release

Do you want to work on this?

Subject to design approval, are you willing to work on a Pull Request for this issue or feature request?

• Yes
• No