Eleminate all warnings

[renemadsen]

We need to eliminate all warnings and implement correct code and still pass tests:

test-dotnet: eFormAPI/eFormAPI.Web/Infrastructure/Models/SelectableList/AdvEntitySelectableGroupEditModel.cs#L36
Non-nullable property 'Description' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the property as nullable.
test-dotnet: eFormAPI/eFormAPI.Web/Infrastructure/Models/SelectableList/AdvEntitySelectableGroupEditModel.cs#L35
Non-nullable property 'GroupUid' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the property as nullable.
test-dotnet: eFormAPI/eFormAPI.Web/Infrastructure/Models/SelectableList/AdvEntitySelectableGroupEditModel.cs#L34
Non-nullable property 'Name' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the property as nullable.
test-dotnet: eFormAPI/eFormAPI.Web/Controllers/Eforms/AudioController.cs#L75
Use IHeaderDictionary.Append or the indexer to append or set headers. IDictionary.Add will throw an ArgumentException when attempting to add a duplicate key. (https://aka.ms/aspnet/analyzers)
test-dotnet: eFormAPI/eFormAPI.Web/Hosting/Security/ClaimsTransformer.cs#L89
Use IHeaderDictionary.Append or the indexer to append or set headers. IDictionary.Add will throw an ArgumentException when attempting to add a duplicate key. (https://aka.ms/aspnet/analyzers)
test-dotnet: eFormAPI/eFormAPI.Web/Hosting/Security/ClaimsTransformer.cs#L80
Use IHeaderDictionary.Append or the indexer to append or set headers. IDictionary.Add will throw an ArgumentException when attempting to add a duplicate key. (https://aka.ms/aspnet/analyzers)
test-dotnet: eFormAPI/eFormAPI.Web/Services/WordProcessor.cs#L62
'HtmlConverter.ParseHtml(string, CancellationToken)' is obsolete: 'Use ParseBody instead for output clarification'
test-dotnet: eFormAPI/eFormAPI.Web/Infrastructure/Models/SelectableList/AdvEntitySelectableGroupEditModel.cs#L36
Non-nullable property 'Description' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the property as nullable.
test-dotnet: eFormAPI/eFormAPI.Web/Infrastructure/Models/SelectableList/AdvEntitySelectableGroupEditModel.cs#L35
Non-nullable property 'GroupUid' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the property as nullable.
test-dotnet: eFormAPI/eFormAPI.Web/Infrastructure/Models/SelectableList/AdvEntitySelectableGroupEditModel.cs#L34
Non-nullable property 'Name' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the property as nullable.