Emit warning about renamed NuGet package on build

[bkoelman]

This helps notify users using the preview packages from GitHub Actions to migrate.

To suppress the warning, add the following to your .csproj:

<PropertyGroup>
    <NoWarn>$(NoWarn);JADNC-OA</NoWarn>
</PropertyGroup>

Contributes to #1578.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.45%. Comparing base (47a1589) to head (f3eb383).

Additional details and impacted files

@@           Coverage Diff            @@
##           openapi    #1596   +/-   ##
========================================
  Coverage    91.45%   91.45%           
========================================
  Files          413      413           
  Lines        13479    13479           
  Branches      2104     2104           
========================================
  Hits         12327    12327           
  Misses         747      747           
  Partials       405      405           

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