Fix compatibility with .NET 5 SDK InternalsVisibleTo

[antonfirsov]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

Same thing as SixLabors/ImageSharp#1337. My Rider is configured with .NET 5 SDK so I need this now.

[antonfirsov]

No idea why does this fail for 2.1 😞

[JimBobSquarePants]

@antonfirsov The issue is unrelated. GitHub have changed the images.
SixLabors/ImageSharp@398e3a7#diff-c2079db070ee682627c591c8361a58c2

We also need to update CodeCov
SixLabors/ImageSharp@f0ed1b7#diff-c2079db070ee682627c591c8361a58c2

[codecov]

Codecov Report

Merging #89 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #89   +/-   ##
=======================================
  Coverage   67.09%   67.09%           
=======================================
  Files          77       77           
  Lines        5079     5079           
  Branches     1056     1056           
=======================================
  Hits         3408     3408           
  Misses       1444     1444           
  Partials      227      227           

Flag	Coverage Δ
#unittests	67.09% <ø> (ø)

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[antonfirsov]

@JimBobSquarePants thanks!

[JimBobSquarePants]

No worries. I should fix Fonts also, will do it this weekend.