Update Src/DSInternals.Common/Data/Hello/KeyCredential.cs

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: MichaelGrafnetter

Src/DSInternals.Common/Data/Hello/KeyCredential.cs

@@ -34,7 +34,7 @@ public class KeyCredential
     /// <summary>
     /// Cached hash of the entries.
     /// </summary>
-    private byte[]? _cachedHash;
+    private readonly byte[]? _cachedHash;
 
     /// <summary>
     /// Defines the version of the structure.