Deprecate direct access to static trait members

Static trait members may only be accessed through a class in which
the trait is used, not directly on the trait.

A complication h

Deprecate direct access to static trait members

Static trait members may only be accessed through a class in which
the trait is used, not directly on the trait.

A complication here is that we should not store static
methods/properties for which a deprecation is triggered in a
cache slot. As the check for this is simple and cheap, I'm handling
this in the cache slot population code in the VM. The alternative
would be to pass the cache slot down into the fetching code.

Part of https://wiki.php.net/rfc/deprecations_php_8_1.

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