Pick right PHP version for sites created before introducing the PHP selector

[fluiddot]

Related to https://github.com/Automattic/studio/pull/240#issuecomment-2173667408 and https://github.com/Automattic/studio/pull/225.

Proposed Changes

• Populate the PHP version for sites without this property specified when loading site details from user data. The PHP version used for those sites is 8.0, as it was the default version used before https://github.com/Automattic/studio/pull/240 got merged.

Testing Instructions

New sites

1. Create a new site.
2. Go to the Settings tab.
3. Observe that the PHP version is 8.1.
4. Open WP admin and navigate to wp-admin/site-health.php?tab=debug.
5. Open the Server section.
6. Observe that the PHP version is 8.1.

Old sites

1. Install a version of the Studio app before changes from https://github.com/Automattic/studio/pull/225 were applied.
   • Alternatively, you can edit the file ~/Library/Application\ Support/Studio/appdata-v1.json and remove phpVersion property from sites.
2. Create a new site.
3. Build/install a version with the changes from this PR.
4. Select the previously created site.
5. Go to the Settings tab.
6. Observe that its PHP version is 8.0.
7. Edit the PHP version.
8. Observe that in the "Edit PHP version" modal, the version 8.0 is selected.

Pre-merge Checklist

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