Update Ubuntu agent on AzP to 20.04

This works around microsoft/artifacts-credprovider#244

Use NuGetAuthenticate in AzP instead of hand installation

Bumps [Nerdbank.GitVersioning](https://github.com/dotnet/Nerdbank.GitVersioning) from 3.4.216 to 3.4.220.
- [Release notes](https://github.com/dotnet/Nerdbank.GitVersioning/releases)
- [Commits](dotnet/Nerdbank.GitVersioning@v3.4.216...v3.4.220)

---
updated-dependencies:
- dependency-name: Nerdbank.GitVersioning
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Add support for TargetFrameworks of the form netXX to Dotnet SDK installer script

Update coverlet.msbuild and .NET SDK

Add support for TargetFrameworks of the form netXX

Bumps [Nerdbank.GitVersioning](https://github.com/dotnet/Nerdbank.GitVersioning) from 3.4.220 to 3.4.231.
- [Release notes](https://github.com/dotnet/Nerdbank.GitVersioning/releases)
- [Commits](dotnet/Nerdbank.GitVersioning@v3.4.220...v3.4.231)

---
updated-dependencies:
- dependency-name: Nerdbank.GitVersioning
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

It is no longer in its LTS lifecycle.

Drop netcoreapp2.1 as a target

Bumps [Microsoft.NET.Test.Sdk](https://github.com/microsoft/vstest) from 16.10.0 to 16.11.0.
- [Release notes](https://github.com/microsoft/vstest/releases)
- [Commits](microsoft/vstest@v16.10.0...v16.11.0)

---
updated-dependencies:
- dependency-name: Microsoft.NET.Test.Sdk
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Bumps [Microsoft.Net.Compilers.Toolset](https://github.com/dotnet/roslyn) from 3.10.0 to 3.11.0.
- [Release notes](https://github.com/dotnet/roslyn/releases)
- [Changelog](https://github.com/dotnet/roslyn/blob/main/docs/Breaking%20API%20Changes.md)
- [Commits](https://github.com/dotnet/roslyn/commits)

---
updated-dependencies:
- dependency-name: Microsoft.Net.Compilers.Toolset
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

This matches what we were already doing on Azure Pipelines.

GitHub workflows update

Update to latest dotnet-install script