Remove Nerdbank.GitVersioning dependency from project in favor of a new
VERSION file, which will be updated ahead of each release. Versioning in
this file will begin with the version we plan to use for the next release:
2.1.0.0.

This change involves the addition of a new custom MSBuild task, which
reads in the contents of the VERSION file, converts it to a Version
object, and then sets the various version-related MSBuild properties with
the correct value (some with the `Revision` component appended, others
without).

Note that there is a bug in MSAL [1] that causes build failures for
projects without dependencies with this change. We add Newtonsoft.Json as
a global dependency in Directory.Build.props to work around this problem
until the fix is released.

[1]: AzureAD/microsoft-authentication-library-for-dotnet#4108

Update release workflow to use VERSION file for versioning, rather than
Nerdbank.GitVersioning.

Remove Nerdbank.GitVersioning tasks (which have been superceded by the use
of the new VERSION file) from the release workflow.

Remove fetch-depth from checkout actions, since that was required by
Nerdbank.GitVersioning. Also take this opportunity to use consistent version
and naming for checkout actions.