Move execution phase of checkstyle

By: md_5 <git@md-5.net>

paper-server/pom.xml

@@ -274,7 +274,7 @@
                         <version>3.0.0</version>
                         <executions>
                             <execution>
-                                <phase>validate</phase>
+                                <phase>process-classes</phase>
                                 <goals>
                                     <goal>check</goal>
                                 </goals>