Provides an abstraction for creating the JavaScriptEncoder used in SystemTextConfigurationEditorJsonSerializer (#19849)

* Provides an abstraction for creating the JavaScriptEncoder used in SystemTextConfigurationEditorJsonSerializer.

* Generalised JSON serialization encoder factory to work for all System.Tex.Json serializers.
Added the serializer's name as a parameter to allow for different encodings per serializer if required.

* Fixed tests by removing use of obsolete constructors.

* Removed name parameter and used a generic type instead.

tests/Umbraco.Tests.UnitTests/Umbraco.Infrastructure/Services/PropertyValidationServiceTests.cs

@@ -41,7 +41,7 @@ public class PropertyValidationServiceTests
             .Returns(new CustomTextOnlyValueEditor(
                 new DataEditorAttribute(Constants.PropertyEditors.Aliases.TextBox),
                 Mock.Of<IShortStringHelper>(),
-                new SystemTextJsonSerializer(),
+                new SystemTextJsonSerializer(new DefaultJsonSerializerEncoderFactory()),
                 Mock.Of<IIOHelper>()));
 
         var propEditors = new PropertyEditorCollection(new DataEditorCollection(() => new[] { dataEditor }));