Warn about using the ignored --mount option

[nirs]

Since #21250 the --mount flag is ignored and --mount-string default was removed. Users that used to run:

minikube start --mount

Should run now:

minikube start --mount-string $HOME/shared/dir

We want to help the user use the new way, and warn them about exposing the home directory to the guest.

Fix

• Detect usage of --mount with unchanged --mount-string and recommend the new way, recommending a specific shared directory instead of $HOME
• Detect usage of --mount with changed --mount-string and warn that the --mount flag is ignored and not needed