I have been trying to diagnose this problem, and I can't figure it out. I'm using Intellij, incidentally.
I have updated spigot, reinstalled my entire server, tried bukkit, reinstalled Intellij, reinstalled my jre, and reinstalled my jdk, and none of it did anything.
[12:26:23 ERROR]: Could not load 'plugins\Test.jar' in folder 'plugins'
org.bukkit.plugin.InvalidPluginException: java.lang.UnsupportedClassVersionError: me/shrimp/test/Core has been compiled by a more recent version of the Java Runtime (class file version 55.0), this version of the Java Runtime only recognizes class file versions up to 52.0
at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:139) ~[spigot-1.15.2.jar:git-Spigot-a03b1fd-bbe3d58]
at org.bukkit.plugin.SimplePluginManager.loadPlugin(SimplePluginManager.java:394) ~[spigot-1.15.2.jar:git-Spigot-a03b1fd-bbe3d58]
at org.bukkit.plugin.SimplePluginManager.loadPlugins(SimplePluginManager.java:301) [spigot-1.15.2.jar:git-Spigot-a03b1fd-bbe3d58]
at org.bukkit.craftbukkit.v1_15_R1.CraftServer.loadPlugins(CraftServer.java:353) [spigot-1.15.2.jar:git-Spigot-a03b1fd-bbe3d58]
at net.minecraft.server.v1_15_R1.DedicatedServer.init(DedicatedServer.java:210) [spigot-1.15.2.jar:git-Spigot-a03b1fd-bbe3d58]
at net.minecraft.server.v1_15_R1.MinecraftServer.run(MinecraftServer.java:784) [spigot-1.15.2.jar:git-Spigot-a03b1fd-bbe3d58]
at java.lang.Thread.run(Unknown Source) [?:1.8.0_241]
Caused by: java.lang.UnsupportedClassVersionError: me/shrimp/test/Core has been compiled by a more recent version of the Java Runtime (class file version 55.0), this version of the Java Runtime only recognizes class file versions up to 52.0
at java.lang.ClassLoader.defineClass1(Native Method) ~[?:1.8.0_241]
at java.lang.ClassLoader.defineClass(Unknown Source) ~[?:1.8.0_241]
at java.security.SecureClassLoader.defineClass(Unknown Source) ~[?:1.8.0_241]
at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:163) ~[spigot-1.15.2.jar:git-Spigot-a03b1fd-bbe3d58]
at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:96) ~[spigot-1.15.2.jar:git-Spigot-a03b1fd-bbe3d58]
at java.lang.ClassLoader.loadClass(Unknown Source) ~[?:1.8.0_241]
at java.lang.ClassLoader.loadClass(Unknown Source) ~[?:1.8.0_241]
at java.lang.Class.forName0(Native Method) ~[?:1.8.0_241]
at java.lang.Class.forName(Unknown Source) ~[?:1.8.0_241]
at org.bukkit.plugin.java.PluginClassLoader.<init>(PluginClassLoader.java:64) ~[spigot-1.15.2.jar:git-Spigot-a03b1fd-bbe3d58]
at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:135) ~[spigot-1.15.2.jar:git-Spigot-a03b1fd-bbe3d58]
... 6 more
It looks to me like the plugin was compiled with a version of java that is greater than what you're using to run the plugin. Either downgrade your plugins java target version used to to compile to match your server or upgrade your server to match your plugins java target version used to compile.