Update caddy installation steps (#5384)

The official installation instructions have changed again.

Author: raulmar0

docs/guide.md

@@ -126,7 +126,7 @@ access code-server on an iPad or do not want to use SSH port forwarding.
 
 ```console
 sudo apt install -y debian-keyring debian-archive-keyring apt-transport-https
-curl -1sLf 'https://dl.cloudsmith.io/public/caddy/stable/gpg.key' | sudo apt-key add -
+curl -1sLf 'https://dl.cloudsmith.io/public/caddy/stable/gpg.key' | sudo gpg --dearmor -o /usr/share/keyrings/caddy-stable-archive-keyring.gpg
 curl -1sLf 'https://dl.cloudsmith.io/public/caddy/stable/debian.deb.txt' | sudo tee /etc/apt/sources.list.d/caddy-stable.list
 sudo apt update
 sudo apt install caddy