Update build scripts: migrate to Spigot API, adjust JVM toolchains and Java version.

Signed-off-by: Chaoscaot <max@maxsp.de>

VelocityCore/Dependencies/build.gradle.kts

@@ -36,6 +36,10 @@ tasks.build {
     finalizedBy(tasks.shadowJar)
 }
 
+kotlin {
+    jvmToolchain(17)
+}
+
 java {
     sourceCompatibility = JavaVersion.VERSION_17
     targetCompatibility = JavaVersion.VERSION_17

WebsiteBackend/build.gradle.kts

@@ -24,6 +24,15 @@ plugins {
     application
 }
 
+kotlin {
+    jvmToolchain(21)
+}
+
+java {
+    sourceCompatibility = JavaVersion.VERSION_21
+    targetCompatibility = JavaVersion.VERSION_21
+}
+
 application {
     mainClass.set("de.steamwar.ApplicationKt")
 }