Update to Minecraft 1.20 (#148)

pom.xml

@@ -29,6 +29,7 @@
     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
     <maven.compiler.source>17</maven.compiler.source>
     <maven.compiler.target>17</maven.compiler.target>
+    <maven.compiler.release>17</maven.compiler.release>
     <!-- Silence IDE warning - property is declared in individual internal modules. -->
     <spigot.version>unknown</spigot.version>
   </properties>
@@ -39,9 +40,8 @@
       <modules>
         <module>api</module>
         <module>plugin</module>
-        <module>internal/v1_18_R2</module>
-        <module>internal/v1_19_R2</module>
         <module>internal/v1_19_R3</module>
+        <module>internal/v1_20_R1</module>
         <module>assembly</module>
       </modules>
     </profile>