[netcore] Move to new arcade post build using yaml stages (mono/mono#16966)

[netcore] Move to new arcade post build using yaml stages (mono/mono#16966)

This is required by the newer arcade builds.
We can simplify our scripts since we no longer need the special publishwitharcade.proj.

Added copyright and license to the .nuspecs since the new arcade validation now
verifies this against a set of Microsoft policies.

We also need to make the .nupkg pre-release by appending the -preview suffix as stable versions are not permitted in non-stable feeds.

Commit migrated from https://github.com/mono/mono/commit/6bc988b0870b7d4d56a73c13b45fe5158f81c64d