fix: update MySQL user creation for MySQL 8.0+ compatibility

Author: henribon

src/main/resources/db/mysql/user.sql

@@ -4,4 +4,8 @@ ALTER DATABASE petclinic
   DEFAULT CHARACTER SET utf8
   DEFAULT COLLATE utf8_general_ci;
 
-GRANT ALL PRIVILEGES ON petclinic.* TO 'petclinic'@'%' IDENTIFIED BY 'petclinic';
+CREATE USER IF NOT EXISTS 'petclinic'@'%' IDENTIFIED BY 'petclinic';
+
+GRANT ALL PRIVILEGES ON petclinic.* TO 'petclinic'@'%';
+
+FLUSH PRIVILEGES;