Fix compatibility with NET 5 SDK InternalsVisibleTo

[JimBobSquarePants]

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Description

.NET 5 bring a native InternalVisibleTo target implementation that is incompatible with our current implementation. The fix is simple, update our property to use the same Key attribute rather than PublicKey.

Tested using Visual Studio 16.8.0 Preview 2.1 and .NET 5 SDK 5.0.100-preview.8.20417.9

cc/ @sandcastle

[codecov]

Codecov Report

Merging #116 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #116   +/-   ##
=======================================
  Coverage   84.64%   84.64%           
=======================================
  Files          46       46           
  Lines        1146     1146           
  Branches      159      159           
=======================================
  Hits          970      970           
  Misses        128      128           
  Partials       48       48           

Flag	Coverage Δ
#unittests	84.64% <ø> (ø)

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Impacted Files	Coverage Δ
...DependencyInjection/ImageSharpBuilderExtensions.cs	69.64% <ø> (ø)

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