housekeeping: Use GitHub Actions

[RLittlesII]

What kind of change does this PR introduce?

Migrates away from Azure Devops to GitHub Actions

What is the current behavior?

Building on Azure Pipelines

What is the new behavior?

building on GitHub Actions

What might this PR break?

The build

Please check if the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information:

[codecov]

Codecov Report

Merging #258 into main will increase coverage by 0.48%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #258      +/-   ##
==========================================
+ Coverage   53.38%   53.87%   +0.48%     
==========================================
  Files          17       23       +6     
  Lines         487      542      +55     
  Branches       80       83       +3     
==========================================
+ Hits          260      292      +32     
- Misses        203      225      +22     
- Partials       24       25       +1     

Impacted Files	Coverage Δ
src/Sextant/Sextant.cs
.../Sextant/System/Reactive/Linq/ToSignalExtension.cs
src/Sextant/Navigation/ViewStackService.cs
src/Sextant/DefaultViewModelFactory.cs
...rc/Sextant/Navigation/ViewModelActionExtensions.cs
src/Sextant.XamForms/SextantHelper.cs
...extant.XamForms/Mixins/DependencyResolverMixins.cs
src/Sextant/ViewModelFactoryNotFoundException.cs
src/Sextant/DependencyResolverMixins.cs
src/Sextant/Navigation/ViewStackServiceBase.cs
... and 30 more

---

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[glennawatson]

Update to your test to be against .net core 3.1 and it should fix one of your build issues.

[glennawatson]

• You'll need to add coverlet msbuild
• Delete cake files
• Delete azure pipelines file
• Remove shouldly
• Update stylecop analyser
• Use DiffEngine API approval from fusillade

[glennawatson]

In a separate pr

• Change GitHub actions badge to readme
• remove azure DevOps from branch pr required actions for approval.
• Add GitHub action build to the branch pr requirements