Change UnixEpoch to field

Author: RenderMichael

dotnet/src/webdriver/LogEntry.cs

@@ -53,7 +53,7 @@ private LogEntry()
         /// </summary>
         public string Message { get; private set; } = string.Empty;
 
-        private static DateTime UnixEpoch => new(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);
+        private static readonly DateTime UnixEpoch = new(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);
 
         /// <summary>
         /// Returns a string that represents the current <see cref="LogEntry"/>.