Document the legacy project upgrade breaking changes

[JanKrivanek]

Background

As part of MSBuild deprecated code removal effort (https://github.com/dotnet/msbuild/issues/8826) an upgrade support from pre-MSBuild projects (and pre-MSBuild webprojects) will be dropped in dev17.12

The document to be updated

https://learn.microsoft.com/en-us/visualstudio/releases/2022/port-migrate-and-upgrade-visual-studio-projects

Previous behavior

When encountered those legacy project types (from .NET 1.1 and prior), Visual Studio was able to automatically run upgrade wizard to make the necessary one-way changes in order to open the projects.

New behavior

Upgrade wizard will run, but it will not be able to convert those old project types (rest of the upgrade will run successfuly). Conversion upgrade log will display a clear customized error, that will provida an aka.ms link for more details:

Once the microsoft.learn article is properly updated (anchor should be added as well) - the aka.ms link will be pointed to the article.

Workaround

Older VS version can be used to run the conversion, after which the upgraded project can be saved and then opened in the new VS version.

Envisioned Timeline

The change is planned for dev17.12 P1

Required changes

• Add section for 'Unsupported upgrade scenarios' - describing the above described behavior and workaround.
• The added section should be directly linkable (so that we can direct to it via the aka.ms link)

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