Update max supported version (#7982)

Brings parity with: https://github.com/NuGet/NuGet.Services.Index/pull/166

Addresses #7984

Author: loic-sharma

src/NuGetGallery/GalleryConstants.cs

@@ -27,7 +27,7 @@ public static class GalleryConstants
         public const int GravatarCacheDurationSeconds = 300;
 
         public const int MaxEmailSubjectLength = 255;
-        internal static readonly NuGetVersion MaxSupportedMinClientVersion = new NuGetVersion("5.5.0.0");
+        internal static readonly NuGetVersion MaxSupportedMinClientVersion = new NuGetVersion("5.6.0.0");
         public const string PackageFileDownloadUriTemplate = "packages/{0}/{1}/download";
 
         public const string ReadMeFileSavePathTemplateActive = "active/{0}/{1}{2}";