Update to supported dotnet

This will remove direct attempts to support running dot net framework apps. It may still partially work in some cases, but using an older version of the converter would be wisest. It also removes the web app's api and cuts it down to a host of static files with the function always providing the api.

Why

It's very hard to find a balance between old and new, particularly around dot net framework and msbuild versions. Instead, we'll target the latest LTS version of dot net and use old builds to satisfy people who turn up with ancient projects that need converting, or advise them to upgrade to the latest dot net first.

Solution

Still need to look into a couple of test failures but wanted to check if this works on CI system.