Increase outdated build delay to 4 weeks

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

paper-server/src/main/java/org/bukkit/craftbukkit/Main.java

@@ -181,7 +181,7 @@ public class Main {
                     Date buildDate = new SimpleDateFormat("yyyyMMdd-HHmm").parse(Main.class.getPackage().getImplementationVendor());
 
                     Calendar deadline = Calendar.getInstance();
-                    deadline.add(Calendar.DAY_OF_YEAR, -21);
+                    deadline.add(Calendar.DAY_OF_YEAR, -28);
                     if (buildDate.before(deadline.getTime())) {
                         System.err.println("*** Error, this build is outdated ***");
                         System.err.println("*** Please download a new build as per instructions from https://www.spigotmc.org/go/outdated-spigot ***");