Support .slnx format

The current versions of the code analysis packages (4.14.0) don't support the newer `.slnx` format for solutions.

Once a package is available on nuget.org that supports this, bump to it and check for `slnx` in the code where we currently check for `.sln`.

Wait for a code analysis package version that supports the .slnx format, then locate the existing code that checks for .sln and review the package references. Update the package version and extend the solution-format check; done means .slnx solutions are handled alongside .sln.

Written by the indexing model from the issue text.