Document migration strategy for tasks that must support older MSBuild versions

Some tasks cannot be retargeted to newer MSBuild versions because, for example, they must remain compatible with older Visual Studio releases. We need clear guidance on how these tasks should migrate to support the multithreaded mode.

**Scope**
- Create a sample/test task and migrate it, demonstrating a migration
- Document migration strategy for tasks that must support older MSBuild versions

Useful links: https://github.com/dotnet/msbuild/blob/98a21fb94d029cd8b9b2244733252db843d73ca7/documentation/specs/multithreading/thread-safe-tasks.md?plain=1#L48