Do not use a default value for MicrosoftNetCompilersToolsetVersion

- [ ] This issue is blocking
- [x] This issue is causing unreasonable pain

It looks like Versions.props [specifies an old version of the Toolset package](https://github.com/dotnet/arcade/blob/ad64a76fd00a641116e79f77d48b42c6e7ed9b0a/eng/Versions.props#L45). 4.8 at time of writing.

What this means is that if user has set property `UsingToolMicrosoftNetCompilers=true`, then this old version of the package is used by default. This is causing the user to use an older compiler than the one in the included .NET SDK, which is not supported.

This should be fixed by deleting the version of the Toolset in Versions.props. This should cause the build to fail when `UsingTool...` is set without also explicitly setting the tool version. Optionally, some more descriptive warning could also be reported in this case, in order to make it more clear how to fix it.