Update to Minecraft 1.21.8, add checkstyle, fix compilation errors

- Update Paper API: 1.20.1 -> 1.21.4
- Update plugin version: 1.5.5-1.20.1 -> 1.5.5-1.21.8
- Add checkstyle plugin with reasonable rules
- Fix Enchantment.LUCK -> Enchantment.UNBREAKING
- Fix EntityType.FIREWORK -> EntityType.FIREWORK_ROCKET
- Remove unused imports
- Add temp_*/ to .gitignore
- 118 files changed, 1633 insertions(+), 1301 deletions(-)

src/main/java/com/loki/lochat/api/service/PlayerDataService.java

@@ -7,5 +7,6 @@ import java.util.UUID;
  */
 public interface PlayerDataService {
     void clearPlayerData(UUID player);
+
     void saveAll();
 }