PHPC-1933: Cursor::key() return type is mixed (#1259)

Using IS_LONG produces a fatal error in PHP 8.1 if the position is invalid and Cursor::key() returns null.

Author: jmikola

src/MongoDB/Cursor.c

@@ -355,7 +355,7 @@ PHP_METHOD(Cursor, rewind)
 ZEND_BEGIN_ARG_WITH_TENTATIVE_RETURN_TYPE_INFO_EX(ai_Cursor_current, 0, 0, IS_MIXED, 0)
 ZEND_END_ARG_INFO()
 
-ZEND_BEGIN_ARG_WITH_TENTATIVE_RETURN_TYPE_INFO_EX(ai_Cursor_key, 0, 0, IS_LONG, 0)
+ZEND_BEGIN_ARG_WITH_TENTATIVE_RETURN_TYPE_INFO_EX(ai_Cursor_key, 0, 0, IS_MIXED, 0)
 ZEND_END_ARG_INFO()
 
 ZEND_BEGIN_ARG_WITH_TENTATIVE_RETURN_TYPE_INFO_EX(ai_Cursor_next, 0, 0, IS_VOID, 0)