fix: raise compiler version

Author: timbru31

pom.xml

@@ -18,8 +18,8 @@
     <plugin.name>SilkSpawnersShopAddon</plugin.name>
     <plugin.main>de.dustplanet.silkspawnersshopaddon.SilkSpawnersShopAddon</plugin.main>
     <plugin.package>de.dustplanet.silkspawnersshopaddon</plugin.package>
-    <jdk.version>1.8</jdk.version>
-    <java.version>1.8</java.version>
+    <jdk.version>21</jdk.version>
+    <java.version>21</java.version>
   </properties>
 
   <repositories>