Update to ImageSharp v3.1.11

[Erik-White]

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 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 updates ImageSharp.Textures to use the latest public release of ImageSharp, version 3.1.11
The license has been updated to the Six Labors Split License

It also updates relevant dependencies:

• SixLabors.SharedInfrastructure submodule to 9a6cf00 to mirror the release/v3.0.x of ImageSharp
• updates formatting and code to conform to rules in SixLabors.SharedInfrastructure (nullability etc)
• updates all test dependencies

I saw the comment that this repo is not maintained and I understand that you don't have time for it, but it would be shame to let all the obvious effort that has gone into this to go to waste! I would like to find a path forward where I (and I'm sure others) can make use of it, as well as contribute, without being a burden on the ImageSharp team.

(Apologies for the size of the diff, the license changes forced a lot of noise with the updated file headers)

[CLAassistant]

All committers have signed the CLA.

[JimBobSquarePants]

Thanks @Erik-White

I would love to get this kick started again once I do the next major releases of the main libraries. I've actually had very little to do with the code so will need to conduct a thorough architectural review following a merge.

[Erik-White]

Great! Thanks for your feedback

[Erik-White]

I have updated the Texture.From* methods to return exceptions instead of nullable types