Document new core22 variable to enable experimental extensions

[giusebar]

What needs to get done

Document and improve messaging explaining that enabling experimental extensions in core22 requires the usage of an environment variable instead of the --enable-experimental-extension flag. The solution could be:

• When running snapcraft with the old flag in core22, add an error message indicating that the flag does not exist anymore.
• Document the behaviour on snapcraft-extensions page.

Why it needs to get done

In Core22, it appears that the process for enabling experimental extensions has been modified. Instead of using the --enable-experimental-extensions flag, it is now required to set the environment variable SNAPCRAFT_ENABLE_EXPERIMENTAL_EXTENSIONS=1 before executing the snapcraft command. However, when running snapcraft with the old --enable-experimental-extensions flag, no error is explicitly reported, simply the the experimental flags are ignored. This makes the process time-consuming and unclear for the developer.