Update all dependency references.

[JimBobSquarePants]

Prerequisites

• I have written a descriptive pull-request title
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• I have provided test coverage for my change (where applicable)

Description

Just a simple update to all reference dependencies. Might help with any .NET FX type forwarding issues

[JimBobSquarePants]

No idea what or why this is happening. Can get the tests to run locally without crashing either because ImageMagick decoder always crashes

https://github.com/SixLabors/ImageSharp/runs/845593121?check_suite_focus=true#step:7:97

[JimBobSquarePants]

@antonfirsov I think we need to reevaluate our usage of RemoteExecutor to workaround some of the flakiness.

https://github.com/SixLabors/ImageSharp/runs/859311527?check_suite_focus=true#step:7:195

It does look like there's a way with Xunit to run code once at startup which would mean we could move it out of static contructors.
https://stackoverflow.com/a/53143426/427899

[antonfirsov]

@JimBobSquarePants the last test failure I see is because of Magic.NET. What if we revert updating that, and address it in a separate PR?

I like the TestFramework trick.

[antonfirsov]

Since it's a test-only change, I don't see a point to keep the 1.0 milestone.

[JimBobSquarePants]

I was going to update the .NET references also. I'll have one last crack in a mo but if that fails I'll scrap it.

[codecov]

Codecov Report

Merging #1262 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1262   +/-   ##
=======================================
  Coverage   82.68%   82.68%           
=======================================
  Files         697      697           
  Lines       30589    30589           
  Branches     3460     3460           
=======================================
  Hits        25294    25294           
  Misses       4593     4593           
  Partials      702      702           

Flag	Coverage Δ
#unittests	82.68% <ø> (ø)

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[JimBobSquarePants]

@antonfirsov Turns out System.Memory is the trigger. I've updated all other .NET dependencies.

[JimBobSquarePants]

I'm just gonna go ahead and merge this now since there are no code changes.