Lower default MTU to 900 (#4985)

Some players continue to have "stuck at connected" issues connecting to
most servers, but apparently this issue has become more prominent in the
last two weeks or so?

It is almost certainly an MTU problem because there are at least two
servers on the hub that run a lower default MTU, and these players had
no problem connecting to them. For one of these reporters, I actually
increased the MTU to 1000 and they could no longer connect, and could
connect again once it was lowered to 900.

It's not clear what recent changes, either to the codebase or to the
public Internet that have been exercising this MTU issue more. For those
experiencing MTU issues, it seems that connecting to a less full server
results in higher probability of success.

Nevertheless, bringing down the default MTU and then possibly enabling
MTU expansion in the future would make this game playable for a small
but not insignificant bit of players.

Robust.Shared/CVars.cs

@@ -69,7 +69,7 @@ namespace Robust.Shared
         /// </summary>
         /// <seealso cref="NetMtuExpand"/>
         public static readonly CVarDef<int> NetMtu =
-            CVarDef.Create("net.mtu", 1000, CVar.ARCHIVE);
+            CVarDef.Create("net.mtu", 900, CVar.ARCHIVE);
 
         /// <summary>
         /// If set, automatically try to detect MTU above <see cref="NetMtu"/>.