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

N.A.

## Current Behaviour

N.A.

## Possible Solution

N.A.

## 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