Use PHP constants from `wp-now`

[fluiddot]

Related to https://github.com/Automattic/studio/pull/225.

Proposed Changes

• Instead of defining new constants for the available and default PHP versions, use the values provided by wp-now and Playground.

Testing Instructions

1. Create a new site.
2. New site should launch with a default version of PHP 8.0.
3. Navigate to the Studio settings for the site.
4. Edit the PHP version to a different PHP version.
5. Observe that the PHP versions listed are the same as the ones offered in https://playground.wordpress.net/.
6. Verify the server restarts and the site is running the new PHP version.

Pre-merge Checklist

• [x] Have you checked for TypeScript, React or other console errors?

[fluiddot]

Thanks, @fluiddot !

I don't think that our default PHP version should be closely coupled to wp-now, but I'll change that in my other PR.

Good point. Yep, I agree there's no need to to use the same value from wp-now.