Modify offline mode warning to include Velocity line (#8812)

This adds a check to the offline-mode warning to see if Velocity is enabled. If it is enabled then it warns the user to make sure to secure their server and links to the Velocity documentation on that.

patches/server/0880-Add-Velocity-IP-Forwarding-Support.patch

@@ -94,6 +94,34 @@ index 0000000000000000000000000000000000000000..c4934979b1ed85bfc4f8d9e6f8848b2b
 +        return buf.readBoolean() ? buf.readUUID() : orElse;
 +    }
 +}
+diff --git a/src/main/java/net/minecraft/server/dedicated/DedicatedServer.java b/src/main/java/net/minecraft/server/dedicated/DedicatedServer.java
+index 51b3db0b6c2cede95b584268e035c0fb36d38094..a7e133f3495e9132a5fdae2c24f225e7b026295a 100644
+--- a/src/main/java/net/minecraft/server/dedicated/DedicatedServer.java
++++ b/src/main/java/net/minecraft/server/dedicated/DedicatedServer.java
+@@ -274,13 +274,20 @@ public class DedicatedServer extends MinecraftServer implements ServerInterface
+         server.enablePlugins(org.bukkit.plugin.PluginLoadOrder.STARTUP);
+         // CraftBukkit end
+ 
++        // Paper start
++        boolean usingProxy = org.spigotmc.SpigotConfig.bungee || io.papermc.paper.configuration.GlobalConfiguration.get().proxies.velocity.enabled;
++        String proxyFlavor = (io.papermc.paper.configuration.GlobalConfiguration.get().proxies.velocity.enabled) ? "Velocity" : "BungeeCord";
++        String proxyLink = (io.papermc.paper.configuration.GlobalConfiguration.get().proxies.velocity.enabled) ? "https://docs.papermc.io/velocity/security" : "http://www.spigotmc.org/wiki/firewall-guide/";
++        // Paper end
+         if (!this.usesAuthentication()) {
+             DedicatedServer.LOGGER.warn("**** SERVER IS RUNNING IN OFFLINE/INSECURE MODE!");
+             DedicatedServer.LOGGER.warn("The server will make no attempt to authenticate usernames. Beware.");
+             // Spigot start
+-            if (org.spigotmc.SpigotConfig.bungee) {
+-                DedicatedServer.LOGGER.warn("Whilst this makes it possible to use BungeeCord, unless access to your server is properly restricted, it also opens up the ability for hackers to connect with any username they choose.");
+-                DedicatedServer.LOGGER.warn("Please see http://www.spigotmc.org/wiki/firewall-guide/ for further information.");
++            // Paper start
++            if (usingProxy) {
++                DedicatedServer.LOGGER.warn("Whilst this makes it possible to use " + proxyFlavor + ", unless access to your server is properly restricted, it also opens up the ability for hackers to connect with any username they choose.");
++                DedicatedServer.LOGGER.warn("Please see " + proxyLink + " for further information.");
++            // Paper end
+             } else {
+                 DedicatedServer.LOGGER.warn("While this makes the game possible to play without internet access, it also opens up the ability for hackers to connect with any username they choose.");
+             }
 diff --git a/src/main/java/net/minecraft/server/network/ServerLoginPacketListenerImpl.java b/src/main/java/net/minecraft/server/network/ServerLoginPacketListenerImpl.java
 index 0bedd9eadbfe1ce290f22d6c648571e25e3ae0e9..093822a5256e8e919350a000239a3e92a2379aaf 100644
 --- a/src/main/java/net/minecraft/server/network/ServerLoginPacketListenerImpl.java