[1.x] Fix some compile time warnings with .NET 8/9

[adrum]

I was getting the following:

/projects/InertiaCore/InertiaCore/Response.cs(66,9): warning ASP0019: 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) /projects/InertiaCore/InertiaCore/InertiaCore.csproj::TargetFramework=net8.0]
/projects/InertiaCore/InertiaCore/Response.cs(67,9): warning ASP0019: 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) /projects/InertiaCore/InertiaCore/InertiaCore.csproj::TargetFramework=net8.0]
/projects/InertiaCore/InertiaCore/Utils/InertiaActionFilter.cs(35,9): warning ASP0019: 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) /projects/InertiaCore/InertiaCore/InertiaCore.csproj::TargetFramework=net8.0]
/projects/InertiaCore/InertiaCore/Extensions/Configure.cs(72,9): warning ASP0019: 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) /projects/InertiaCore/InertiaCore/InertiaCore.csproj::TargetFramework=net8.0]
/projects/InertiaCore/InertiaCore/Utils/LocationResult.cs(17,13): warning ASP0019: 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) /projects/InertiaCore/InertiaCore/InertiaCore.csproj::TargetFramework=net8.0]

If/when accepted, it could also be merged into main.