* Fix handling of repo analyzers and warnings-as-errors

When we brought in the new SDK, it enabled analyzers by default (which then used our custom ruleset), but a bunch of projects (in particular tests) weren't expecting that, such that we now have thousands of warnings in the repo. This opts-out those projects.

It also enables warnings-as-errors at the root level of the repo, to hopefully avoid such warning storms in the future, and to also clean up the remaining that exist.  This includes a bunch of new obsoletion and platform compat warnings that are firing in the runtime tests.

We may choose to run analyzers on additional projects in the future where it's currently disabled, but this gets us back to a state at least as good if not better than we were previously.

* Fix analyzer warnings on Microsoft.NET.HostModel

Fixes the warnings that were triggered by our rule set applying to this project.  All fixes were automated.

* Fix analyzer warnings in additional projects

* Remove several `<RunAnalyzers>false</RunAnalyzers>`

* Try to opt-out remaining coreclr tests
Co-authored-by: NDavid Mason <davmason@microsoft.com>