Shorten the actual KSpigot setup guide by outsourcing common requirements

guide/docs/setup/beginners/java_version.md

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+Configuring the Java version is nothing specific to KSpigot, it should always be done. It is listed in this guide
+anyways, because a lot of beginners forget to do this - and then get confused about not being able to use inline
+functions.
+
+**Java 16** is the minimum required Java version by Minecraft, therefore KSpigot requires it as well.
+
+You can configure the Java version using Gradle:
+
+```kotlin
+// set the Java version you are using, Java 16 is the minimum required version for Minecraft
+
+tasks.compileJava {
+    options.release.set(16)
+}
+
+tasks.compileKotlin {
+    kotlinOptions.jvmTarget = "16"
+}
+```