.Net: Remove EditorBrowsable(EditorBrowsableState.Never) from ChatMessageContent.Content property (microsoft#10847)

### Motivation, Context and Description
This PR removes the `EditorBrowsable(EditorBrowsableState.Never)`
attribute from the `ChatMessageContent.Content` property because it is
confusing and can be handy to set text content with a one-liner:
`cmc.Content = "some content"`.

Author: SergeyMenshykh

dotnet/src/SemanticKernel.Abstractions/Contents/ChatMessageContent.cs

@@ -1,7 +1,6 @@
 // Copyright (c) Microsoft. All rights reserved.
 
 using System.Collections.Generic;
-using System.ComponentModel;
 using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Text;
@@ -34,7 +33,6 @@ public string? AuthorName
     /// <summary>
     /// A convenience property to get or set the text of the first item in the <see cref="Items" /> collection of <see cref="TextContent"/> type.
     /// </summary>
-    [EditorBrowsable(EditorBrowsableState.Never)]
     [JsonIgnore]
     public string? Content
     {