Update to .NET 6 and ImageSharp v3.x

[JimBobSquarePants]

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

This PR touches a lot of files (I'm sorry) but was the absolute minimum I could do in order to upgrade the project and fix all the build warnings.

It's all cosmetic. Basically

• Updates target frameworks and dependencies
• Updates shared infastructure
• Fixes API changes and additional build warnings
• Updates license, readme, and, ci builds

Note. I copied a bunch of test class updates directly from ImageSharp. We've got a lot of stuff we can remove in our test utilities like reference codecs but I'll leave that for another time.

[codecov]

Codecov Report

Merging #286 (ae40bad) into main (b319910) will decrease coverage by 1%.
The diff coverage is 84%.

@@         Coverage Diff         @@
##           main   #286   +/-   ##
===================================
- Coverage    80%    80%   -1%     
===================================
  Files        97     99    +2     
  Lines      5024   5055   +31     
  Branches    919    919           
===================================
+ Hits       4029   4050   +21     
- Misses      796    807   +11     
+ Partials    199    198    -1     

Flag	Coverage Δ
unittests	80% <84%> (-1%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
src/ImageSharp.Drawing/Processing/Brush.cs	0% <0%> (ø)
...mageSharp.Drawing/Shapes/PolygonClipper/BoundsF.cs	0% <0%> (ø)
...rp.Drawing/Shapes/PolygonClipper/PolygonClipper.cs	68% <ø> (ø)
....Drawing/Shapes/PolygonClipper/PolygonOffsetter.cs	73% <ø> (ø)
...arp.Drawing/Shapes/Rasterization/ActiveEdgeList.cs	98% <ø> (ø)
...arp.Drawing/Shapes/Rasterization/PolygonScanner.cs	96% <ø> (ø)
...awing/Shapes/Rasterization/RasterizerExtensions.cs	100% <ø> (ø)
...mageSharp.Drawing/Shapes/Rasterization/ScanEdge.cs	94% <ø> (ø)
...g/Shapes/Rasterization/ScanEdgeCollection.Build.cs	95% <ø> (+<1%)	⬆️
...Drawing/Shapes/Rasterization/ScanEdgeCollection.cs	86% <ø> (ø)
... and 89 more

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