Re-enable IDE0031 in VMR when null-propagation in l-values feature gets into winforms repo (SDK p4)

[Tanya-Solyanik]

VMR is using P4 for dotnet and the SDK and gets support for ? in L-values feature.
We have IDE0031 enabled as a warning and warnings are treated as errors. New P4 analyzers suggest changing code

            if (pceltFetched is not null)
            {
                pceltFetched[0] = 0;
            }

to

pceltFetched?[0] = 0;

in VMR and build fails, but winforms repo build does not have these features yet. We'll update code to support the new features when we get P4.

Originally posted by @NikolaMilosavljevic in #13260 (comment)