Update supported GameVersions

src/Impostor.Server/Net/Manager/ClientManager.cs

@@ -20,8 +20,7 @@ namespace Impostor.Server.Net.Manager
     {
         private static readonly HashSet<int> SupportedVersions = new HashSet<int>
         {
-            GameVersion.GetVersion(2021, 3, 25), // 2021.3.31s
-            GameVersion.GetVersion(2021, 4, 2), // 2021.4.2a
+            GameVersion.GetVersion(2021, 4, 25), // 2021.6.15
         };
 
         private readonly ILogger<ClientManager> _logger;