Add s390x support if NET6_0_OR_GREATER

Added support for s390x architecture in Rid property.

Author: Jawahars

Rubjerg.Graphviz/GraphvizCommand.cs

@@ -27,7 +27,9 @@ internal static string Rid
                 Architecture.Arm64 => "arm64",
                 Architecture.X86 => "x86",
                 Architecture.Arm => "arm",
-                Architecture.S390x => "s390x",
+#if NET6_0_OR_GREATER
+                Architecture.S390x => "s390x",  // s390x support added in .NET 6
+endif
                 _ => "unknown"
             };
 
@@ -132,3 +134,4 @@ public static (byte[] stdout, string stderr) Exec(Graph input, string format = "
 }
 
 
+