Migrate to supported version of .NET

[iblazhko]

Description

Test projects in the solution target .NET Core 2.2 (netcoreapp2.2).

.NET Core 2.2 has reached end of life and is no longer supported.

Proposal

Migrate to .NET Core 3.1 or .NET 5.0
Ideally when .NET 6.0 is out, BO should use that because 6.0 is LTS.

Benefits

1. Makes contribution easier, no need to make local changes that should not go in a PR or install an outdated .NET SDK just for BO work

Risks

No risks as such, public packages may still target netstandard2.0, so no changes for external consumers.