Hello I have a PaperMC Server runing on Linux with Java 21 nad when trying to start the server with the pligin the following erro gets thrown:
Server thread/ERROR]: [ModernPluginLoadingStrategy] Could not load plugin 'VillagerSaver.jar' in folder 'plugins/.paper-remapped' org.bukkit.plugin.InvalidPluginException: java.lang.UnsupportedClassVersionError: cl/mariofinale/VillagerSaver has been compiled by a more recent version of the Java Runtime (class file version 67.0), this version of the Java Runtime only recognizes class file versions up to 65.0 at io.papermc.paper.plugin.provider.type.spigot.SpigotPluginProvider.createInstance(SpigotPluginProvider.java:129) ~[paper-1.21.1.jar:1.21.1-131-84281ce] at io.papermc.paper.plugin.provider.type.spigot.SpigotPluginProvider.createInstance(SpigotPluginProvider.java:35) ~[paper-1.21.1.jar:1.21.1-131-84281ce] at io.papermc.paper.plugin.entrypoint.strategy.modern.ModernPluginLoadingStrategy.loadProviders(ModernPluginLoadingStrategy.java:116) ~[paper-1.21.1.jar:1.21.1-131-84281ce] at io.papermc.paper.plugin.storage.SimpleProviderStorage.enter(SimpleProviderStorage.java:38) ~[paper-1.21.1.jar:1.21.1-131-84281ce] at io.papermc.paper.plugin.entrypoint.LaunchEntryPointHandler.enter(LaunchEntryPointHandler.java:40) ~[paper-1.21.1.jar:1.21.1-131-84281ce] at org.bukkit.craftbukkit.CraftServer.loadPlugins(CraftServer.java:547) ~[paper-1.21.1.jar:1.21.1-131-84281ce] at net.minecraft.server.dedicated.DedicatedServer.initServer(DedicatedServer.java:292) ~[paper-1.21.1.jar:1.21.1-131-84281ce] at net.minecraft.server.MinecraftServer.runServer(MinecraftServer.java:1214) ~[paper-1.21.1.jar:1.21.1-131-84281ce] at net.minecraft.server.MinecraftServer.lambda$spin$0(MinecraftServer.java:329) ~[paper-1.21.1.jar:1.21.1-131-84281ce] at java.base/java.lang.Thread.run(Thread.java:1583) ~[?:?] Caused by: java.lang.UnsupportedClassVersionError: cl/mariofinale/VillagerSaver has been compiled by a more recent version of the Java Runtime (class file version 67.0), this version of the Java Runtime only recognizes class file versions up to 65.0 at java.base/java.lang.ClassLoader.defineClass1(Native Method) ~[?:?] at java.base/java.lang.ClassLoader.defineClass(ClassLoader.java:1027) ~[?:?] at java.base/java.security.SecureClassLoader.defineClass(SecureClassLoader.java:150) ~[?:?] at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:243) ~[paper-mojangapi-1.21.1-R0.1-SNAPSHOT.jar:?] at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:593) ~[?:?] at org.bukkit.plugin.java.PluginClassLoader.loadClass0(PluginClassLoader.java:169) ~[paper-mojangapi-1.21.1-R0.1-SNAPSHOT.jar:?] at org.bukkit.plugin.java.PluginClassLoader.loadClass(PluginClassLoader.java:164) ~[paper-mojangapi-1.21.1-R0.1-SNAPSHOT.jar:?] at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:526) ~[?:?] at java.base/java.lang.Class.forName0(Native Method) ~[?:?] at java.base/java.lang.Class.forName(Class.java:534) ~[?:?] at java.base/java.lang.Class.forName(Class.java:513) ~[?:?] at org.bukkit.plugin.java.PluginClassLoader.<init>(PluginClassLoader.java:78) ~[paper-mojangapi-1.21.1-R0.1-SNAPSHOT.jar:?] at io.papermc.paper.plugin.provider.type.spigot.SpigotPluginProvider.createInstance(SpigotPluginProvider.java:125) ~[paper-1.21.1.jar:1.21.1-131-84281ce] ... 9 more

From that error it seem Java 23 is needed but the Server does not start at all with Java 23 for some other issues.

It can be compiled for java 1.21 but for the meantime version 1.4.1 of the plugin should work.

Fixed on 1.4.3