Pin ubuntu build to 22.04 due to .NET 5 issue

[paulirwin]

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Pins the ubuntu builds to Ubuntu 22.04 LTS due to a .NET 5 OpenSSL dependency issue. We should find a better approach sometime before 22.04 goes out of support in 2027.

Fixes #970

Description

Changes the build agent from ubuntu-latest to ubuntu-22.04.

[NightOwl888]

We should find a better approach sometime before 22.04 goes out of support in 2027.

We will have a better approach soon because we will be able to drop the net6.0 target and use it instead of net5.0 for testing netstandard2.1.

Looks like the azure-pipelines.yml wasn't included, but let's do that because Azure DevOps uses the same images. For that one, we can easily limit this change to net5.0.

[NightOwl888]

Looks good. I confirmed the linux net5.0 tests are producing results on both Azure DevOps and GitHub Actions.