fix: mark FeedbackBase default constructor as obsolete

There are situations now where feedbacks in the feedback collection
can be used to update things on UIs. If the feedback doesn't have a key, it can't
be used for this purpose.

src/PepperDash.Essentials.Core/Feedbacks/BoolFeedback.cs

@@ -42,6 +42,7 @@ namespace PepperDash.Essentials.Core
         /// it will NOT reflect an actual value from a device until <seealso cref="FireUpdate"/> has been called
         /// </remarks>
         /// <param name="valueFunc">Delegate to invoke when this feedback needs to be updated</param>
+        [Obsolete("use constructor with Key parameter. This constructor will be removed in a future version")]
         public BoolFeedback(Func<bool> valueFunc)
             : this(null, valueFunc)
         {

src/PepperDash.Essentials.Core/Feedbacks/FeedbackBase.cs

@@ -9,8 +9,14 @@ using PepperDash.Core;
 
 namespace PepperDash.Essentials.Core
 {
+    /// <summary>
+    /// Base class for all feedback types
+    /// </summary>
     public abstract class Feedback : IKeyed
     {
+        /// <summary>
+        /// Occurs when the output value changes
+        /// </summary>
         public event EventHandler<FeedbackEventArgs> OutputChange;
 
         /// <summary>
@@ -44,10 +50,13 @@ namespace PepperDash.Essentials.Core
         /// <summary>
         /// Base Constructor - empty
         /// </summary>
-		protected Feedback()
-		{
-		}
+        [Obsolete("use constructor with Key parameter. This constructor will be removed in a future version")]
+        protected Feedback() : this(null) { }
 
+        /// <summary>
+        /// Constructor with Key parameter
+        /// </summary>
+        /// <param name="key">The key for the feedback</param>
         protected Feedback(string key)
         {
             if (key == null)

src/PepperDash.Essentials.Core/Feedbacks/SerialFeedback.cs

@@ -25,6 +25,7 @@ namespace PepperDash.Essentials.Core
 
         List<StringInputSig> LinkedInputSigs = new List<StringInputSig>();
 
+        [Obsolete("use constructor with Key parameter. This constructor will be removed in a future version")]
         public SerialFeedback()
         {
         }

src/PepperDash.Essentials.Core/Feedbacks/StringFeedback.cs

@@ -38,6 +38,7 @@ namespace PepperDash.Essentials.Core
         /// it will NOT reflect an actual value from a device until <seealso cref="FireUpdate"/> has been called
         /// </remarks>
         /// <param name="valueFunc">Delegate to invoke when this feedback needs to be updated</param>
+        [Obsolete("use constructor with Key parameter. This constructor will be removed in a future version")]
         public StringFeedback(Func<string> valueFunc)
             : this(null, valueFunc)
         {