Backport for compatibility with v3.0 branch

[binary1230]

This is a minor sync-up to try and keep the 2.0 branch (stable) and the 3.0 branch (dev) in sync a bit better.

There's another branch that is the opposite (merging in latest changes from 2.0 into 3.0).

notes:

• everything in here is more meant to match renames/etc for where things ended up in the 3.0 branch, no major changes (even if the diff looks big)
• one real change: switch from the older "MSbuild style" .csproj format to newer "SDK style" projects
• doesn't upgrade to .net5 but that's a piece of cake to port from the 3.0 branch