Reject strict mixed when a template type is expected

[klimick]

At level 9 and above, passing explicitly typed mixed to callable(A): A should produce an error because the value is not guaranteed to match A.
This change adds that diagnostic.

Below level 9, passing mixed to callable(A): A is allowed. This behavior remains unchanged.

Closes phpstan/phpstan#11935

[klimick]

This PR makes PHPStan report an error for code like this:

/**
* @phpstan-template TValue
*/
final class NonThreadSafeValue extends ThreadSafe
{
   /**
    * @phpstan-return TValue
    *
    * ERROR: Method NonThreadSafeValue::deserialize() should return TValue but returns mixed.
    */
   public function deserialize(): mixed
   {
       return igbinary_unserialize($this->variable);
   }
}

This is technically correct because mixed is not guaranteed to be the template type TValue.
Should this stricter behavior be enabled only under bleedingEdge or behind a separate configuration option?

P.S. I noticed this while investigating the failed PMMP integration job in CI.
P.S.S The failed jobs are unrelated to this change, except for PMMP.