Adds missing SQL upgrade script

* Fixes #4757.
* Adds SQL script to give players back skill points from removed skills.
Thanks to @reunite-ro!

sql-files/upgrades/upgrade_20200402.sql

@@ -0,0 +1,16 @@
+-- AB_EUCHARISTICA
+UPDATE `char` c, `skill` s SET `c`.skill_point = `c`.skill_point + `s`.lv WHERE `s`.id = 2049 AND `c`.char_id = `s`.char_id;
+DELETE FROM `skill` WHERE `id` = 2049;
+
+-- GN_SLINGITEM
+UPDATE `char` c, `skill` s SET `c`.skill_point = `c`.skill_point + `s`.lv WHERE `s`.id = 2493 AND `c`.char_id = `s`.char_id;
+DELETE FROM `skill` WHERE `id` = 2493;
+
+-- GN_MAKEBOMB
+UPDATE `char` c, `skill` s SET `c`.skill_point = `c`.skill_point + `s`.lv WHERE `s`.id = 2496 AND `c`.char_id = `s`.char_id;
+DELETE FROM `skill` WHERE `id` = 2496;
+
+-- ONLY RUN THE BELOW QUERIES IF YOU ARE ON RENEWAL
+-- CR_CULTIVATION
+UPDATE `char` c, `skill` s SET `c`.skill_point = `c`.skill_point + `s`.lv WHERE `s`.id = 491 AND `c`.char_id = `s`.char_id;
+DELETE FROM `skill` WHERE `id` = 491;