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(-)

lovelocity/build.gradle.kts

@@ -1,5 +1,6 @@
 plugins {
     java
+    checkstyle
 }
 
 repositories {
@@ -12,7 +13,7 @@ dependencies {
     compileOnly("com.velocitypowered:velocity-api:3.3.0-SNAPSHOT")
     annotationProcessor("com.velocitypowered:velocity-api:3.3.0-SNAPSHOT")
     implementation("com.google.code.gson:gson:2.10.1")
-    
+
     testImplementation("org.junit.jupiter:junit-jupiter-api:5.10.0")
     testImplementation("org.junit.jupiter:junit-jupiter-engine:5.10.0")
     testRuntimeOnly("org.junit.platform:junit-platform-launcher:1.10.0")
@@ -23,3 +24,17 @@ tasks.jar {
     from(configurations.runtimeClasspath.get().map { if (it.isDirectory) it else zipTree(it) })
     duplicatesStrategy = DuplicatesStrategy.EXCLUDE
 }
+
+checkstyle {
+    toolVersion = "10.12.5"
+    configFile = file("${rootProject.rootDir}/config/checkstyle/checkstyle.xml")
+    isIgnoreFailures = false
+    maxWarnings = 0
+}
+
+tasks.withType<Checkstyle> {
+    reports {
+        xml.required.set(false)
+        html.required.set(true)
+    }
+}