Fix nullable property in csproj files

[bartdesmet]

Future is <Nullable> so changing those. See dotnet/roslyn#35432.

[codecov]

Codecov Report

Merging #938 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #938   +/-   ##
=======================================
  Coverage   65.43%   65.43%           
=======================================
  Files         165      165           
  Lines       25233    25233           
  Branches    18210    18210           
=======================================
  Hits        16510    16510           
  Misses       2065     2065           
  Partials     6658     6658

Flag	Coverage Δ
#ixnet	65.43% <ø> (ø)	⬆️

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[clairernovotny]

Closing as out of date