Outdated SixLabors.ImageSharp usage

[ynot01]

https://github.com/Nihlus/Puzzle/blob/master/Puzzle/SignatureGenerator.cs#L161

if (!image.TryGetSinglePixelSpan(out var pixels))

The method TryGetSinglePixelSpan was changed as noted in this issue

Screenshot below

Because of this, trying to use Puzzle's GenerateSignature method results in the following exception

System.MissingMethodException: Method not found: 'Boolean SixLabors.ImageSharp.Image`1.TryGetSinglePixelSpan(System.Span`1<!0> ByRef)'.
   at Puzzle.SignatureGenerator.ComputeAverageSampleLuminosities(Image`1 image)
   at Puzzle.SignatureGenerator.GenerateSignature(Image`1 image)