Fixes user creation in SQL

The user creation defined in the SQL files was valid for MySQL 5, but
it is no longer supported by MySQL 8+. This commit fixes this error
by using a syntaxis compatible with MySQL 8+.

additional-material/db/xcs-sample-mysql.sql

@@ -31,7 +31,8 @@ CREATE TABLE `Pet` (
 --
 -- User creation
 --
-GRANT ALL PRIVILEGES ON xcs.* TO xcs@localhost IDENTIFIED BY 'xcs'; 
+CREATE USER IF NOT EXISTS xcs@localhost IDENTIFIED BY 'xcs';
+GRANT ALL PRIVILEGES ON xcs.* TO xcs@localhost; 
 FLUSH PRIVILEGES;
 
 --

additional-material/db/xcs-sample-test-mysql.sql

@@ -31,5 +31,6 @@ CREATE TABLE `Pet` (
 --
 -- User creation
 --
-GRANT ALL PRIVILEGES ON xcssampletest.* TO xcs@localhost IDENTIFIED BY 'xcs'; 
+CREATE USER IF NOT EXISTS xcs@localhost IDENTIFIED BY 'xcs';
+GRANT ALL PRIVILEGES ON xcssampletest.* TO xcs@localhost; 
 FLUSH PRIVILEGES;
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