This includes a sidecar XML file which will suppress new warnings in
the 6.0 framework.

Note that a few of the new warnings for compiler-generated code are
suppressed in 7.0 with attributes on local functions. In the XML I had
to suppress them with an attribute on the user method (since the
signature format doesn't support compiler-generated code).

Also note that this approach only includes suppressions for the core
framework, not OOB assemblies. OOB assemblies or third-party nuget
packages will require a slightly different approach to avoid warnings
about unresolved members referenced in XML, in apps where those
assemblies aren't used. There are no warnings that we need to suppress
in OOB assemblies - only new warnings in already trim-incompatible
code.

This also includes a fix for a varargs method issue I found while
running on the 6.0 framework.