2.1 has fallen out of its LTS lifetime, and the docker images have been removed.

Drop .NET Core 2.1 SDK support

Apply `is null` pattern and analyzer

This demonstrates the repro for #658

The managed git implementation had it correctly implemented, according to the code comments in the libgit2 implementation. But those code comments did not match the *code* in the libgit2 implementation. As a result, the managed git code did not match.
In this commit I correct the libgit2 impl code comments to describe what the code *actually* does. Then I fix the identical code comment and code in the managed git implementation to match.

Fixes #658

This is the latest version that still supports netcoreapp3.1.
This should fix the problem of parsing msbuild project files with metadata as xml attributes.

…String() throws

Fixes #588

Fix `nbgv` dependencies

Count merge commits in path filtered height if any parent changed it

…re-release`

Fixes #630

Suppress `cloudBuild.buildNumber: null` being written for `nbgv prepare-release`

…d-variable-retry

Add retry to SetCloudBuildVariable method in VisualStudioTeamServices