Add a optional reason for the requires msbuild theory/fact attribute

[marcpopMSFT]

In prepping the warning for net9 on 17.11, I ended up having to update a bunch of tests to require 17.12. I noticed that all of the tests previously required 17.8 and so potentially hit the same exact issue last year. It'd be nice to have an optional parameter that we could potentially print out that would report why the minimum value is what it is.

c30463d

[dotnet-issue-labeler]

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[dotnet-issue-labeler]

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