Change levelUp method to remove duplication

src/main/java/re/forestier/edu/rpg/Adventurer.java

src/main/java/re/forestier/edu/rpg/Archer.java

src/main/java/re/forestier/edu/rpg/Dwarf.java

src/main/java/re/forestier/edu/rpg/Goblin.java

src/main/java/re/forestier/edu/rpg/Player.java