Use portable PDBs even on .NETFramework (#42331)

Use portable PDBs even on .NETFramework (#42331)

Previously we were building full / windows PDBs for .NETFramework
because at the time .NETFramework couldn't read portable PDBs
so things like StackTrace wouldn't include file/line info.

That has been fixed as of 4.7.2 and we can switch to portable PDB
everywhere.

Symbol archiving will still convert to windows PDBs where necessary so
that we don't break any consumption scenarios for folks on older
frameworks.

This is important to get the latest symbols features only available in
the portable PDB format.