Mark class aliases as deprecated in a different way

[oliverklee]

Our current way of marking the class aliases as deprecated doesn't get registered with PHPStan (and also not with my IDE). So we'll need a different approach.

https://www.reddit.com/r/PHP/comments/51riss/using_class_alias_to_maintain_bc_while/

[JakeQZ]

I tried the Reddit-suggested solution in #1559, but it doesn't seem to work:

• It appears that PHPStan can see that the if statement condition will always evaluate to false, so appears to still not pick up the deprecation notice (I tried swapping out the new class name for the old one in Emogrifier, and did not get any additional errors from Stan);
• It causes additional warnings from Stan in this project: Negated boolean expression is always true. 🪪 booleanNot.alwaysTrue