Re-target Msix.Utils package to .NET Standard 2.0

[florelis]

This re-targets the Microsoft.Msix.Utils library to .NET Standard 2.0 (from .NET Framework 4.6). .NET Standard 2.0 is supported by .NET Framework 4.6.1 and .NET Core 2.0 or higher, which are used by all known existing consumers of the package. This may be a breaking change, so I increased the major version of the package.

Changes:

• Updated the project's csproj files to target .NET Standard for the library and .NET 5 for the test projects.
• This change required updating several depencencies:
  • Win32.Registry now comes from a NuGet package. Win32.Registry and Diagnostics.Tracing are now under a different namespace.
  • Moved code analysis from StyleCop to StyleCop.Analyzers as StyleCop is no longer actively maintained. I had to do several style changes to appease StyleCop:
    • Added <see /> tags to doc comments in constructors
    • Removed [SuppressMessage] attributes for missing documentation in interop, and moved it to a single .editorconfig file.
    • Added this. in some places
    • Removed some trailing whitespace
    • For other more common violations, I suppressed the warning in the .editorconfig instead to reduce number of changes.
  • Moved tests from TAEF to MSTest which is more common and required little code changes.
    • Updated pipeline from calling te.exe to use VSTest task.
    • Moved test data files to UtilsTests directory.
    • Most code changes needed were direct replacements: Verify -> Assert, new usings, Log.Comment -> LogMessage (using static directly instead of Logger.LogMessage to avoid name collision with Logger).
    • Made test classes public as required by MSTest.
    • Fixed a bug in the Logger where the Formatter was not reset during Deinit() and that appeared due to tests running in a different order.
• Removed unused x86/x64 platforms for the test project as only AnyCPU is used.
• Also updated the file copyright headers as per requirements.

See #399