Error because Nashorn was removed for java15+

[pianwan]

Mojang is now porting Minecraft to java16. We are updating to java16 and suffer a problem with this plugin.

[12:55:39] [Server thread/WARN]: [MythicMobsExtension] Task #206 for MythicMobsExtension v1.737c-SNAPSHOT generated an exception java.lang.NoClassDefFoundError: jdk/nashorn/api/scripting/NashornScriptEngineFactory at com.gmail.berndivader.mythicmobsext.javascript.Nashorn.(Nashorn.java:40) ~[?:?] at com.gmail.berndivader.mythicmobsext.javascript.JavaScript$1.run(JavaScript.java:13) ~[?:?] at org.bukkit.craftbukkit.v1_16_R3.scheduler.CraftTask.run(CraftTask.java:100) ~[patched_1.16.5.jar:git-Paper-759] at org.bukkit.craftbukkit.v1_16_R3.scheduler.CraftScheduler.mainThreadHeartbeat(CraftScheduler.java:468) ~[patched_1.16.5.jar:git-Paper-759] at net.minecraft.server.v1_16_R3.MinecraftServer.w(MinecraftServer.java:1078) ~[patched_1.16.5.jar:git-Paper-759] at net.minecraft.server.v1_16_R3.MinecraftServer.lambda$a$0(MinecraftServer.java:291) ~[patched_1.16.5.jar:git-Paper-759] at java.lang.Thread.run(Thread.java:831) [?:?] Caused by: java.lang.ClassNotFoundException: jdk.nashorn.api.scripting.NashornScriptEngineFactory at org.bukkit.plugin.java.PluginClassLoader.loadClass0(PluginClassLoader.java:153) ~[patched_1.16.5.jar:git-Paper-759] at org.bukkit.plugin.java.PluginClassLoader.loadClass(PluginClassLoader.java:114) ~[patched_1.16.5.jar:git-Paper-759] at java.lang.ClassLoader.loadClass(ClassLoader.java:519) ~[?:?] ... 7 more