Differentiate "max" and "max supported" PHP versions

Introduce a new PHP_SUPPORTED_VERSION constant and add a comment block
to explain usage of the 3 PHP-version-related constants.

Enforce PHP_MAX_VERSION in core.php, like we do for PHP_MIN_VERSION.

Adapt the Admin Check to use PHP_SUPPORTED_VERSION; check is downgraded
to warning since the hard fail is already taken care of.

PHP_SUPPORTED_VERSION should be defined with 2 numbers (X.Y) to cover
all versions including pre-releases.

Fixes 0036540