Downgrade PHP version from 8.0 to 5.6 for development builds

[westonruter]

See https://wordpress.org/support/topic/version-2-2-2-parse-error-syntax-error-unexpected/

[github-actions]

Plugin builds for e880012 are ready 🛎️!

• Download development build
• Download production build

[westonruter]

To prevent the support topic issue from coming up again:

1. We should run phpcs PHP version compatibility checks on all files in the vendor directory, or:
2. Use the build ZIP from GHA instead of the version built locally and also ensure that the version of PHP used for building always matches the minimum version supported.