🍎Update key-availability command on macOS (#21288)

Co-authored-by: Jess Hosman <1183847+jhosman@users.noreply.github.com>

content/developers/overview/using-ssh-agent-forwarding.md

@@ -155,9 +155,11 @@ $ ssh-add YOUR-KEY
 On macOS, `ssh-agent` will "forget" this key, once it gets restarted during reboots. But you can import your SSH keys into Keychain using this command:
 
 ```shell
-$ ssh-add -K YOUR-KEY
+$ ssh-add --apple-use-keychain YOUR-KEY
 ```
 
+For MacOS versions prior to Monterey (12.0), use `-K` instead of `--apple-use-keychain`. For more information, see "[Adding your SSH key to the ssh-agent](/authentication/connecting-to-github-with-ssh/generating-a-new-ssh-key-and-adding-it-to-the-ssh-agent#adding-your-ssh-key-to-the-ssh-agent)."
+
 {% endtip %}
 
 [tech-tips]: http://www.unixwiz.net/techtips/ssh-agent-forwarding.html