Core/Pets: Change PET_SAVE_NOT_IN_SLOT constant from 100 to -1 to truly avoid conflicts when growing stable size and change PET_SAVE_AS_DELETED from -1 to -2

sql/updates/characters/master/2022_01_31_02_characters.sql

@@ -0,0 +1,3 @@
+ALTER TABLE `character_pet` CHANGE `slot` `slot` smallint NOT NULL DEFAULT '-1';
+
+UPDATE `character_pet` SET `slot` = -1 WHERE `slot` = 100;