[FEAT] README: Mark sysctl as always required

[hoang-himself]

Is this a new feature request?

• I have searched the existing issues

Wanted change

Sysctl parameter should be required in both server and client mode
https://github.com/linuxserver/docker-wireguard/blob/master/readme-vars.yml#L40

Reason for change

I was having problems with this image using rootless Podman, and I found that the sysctl parameter is necessary in server mode
I have not tried with rootful Podman or Docker, please advise otherwise

Proposed code change

desc: "Required."

[github-actions]

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[aptalca]

It is not always required