Clean up .NET 5 references and update to WinAppSDK projection for testing

[manodasanW]

• Build pipelines recently started failing due to the x86 version of .NET 5 doesn't seem to be installed by default anymore. I took that opportunity to clean up the .NET 5 references we had rather than doing a manual install. This should also help improve our build times given we are building one less TFM.
• One of the things blocking the cleanup of the .NET 5 TFM was we still used the old WinUI packages for our testing and they ran into build issues with .NET 6. This PR moves us to using a recent version of WinAppSDK for testing which does build without issues and also addresses the breaking changes from the old WinUI packages to WinAppSDK. We still build a projection for the old version to help with catching any regressions.
• Fixing issue with build.cmd after one of the previous PRs

Fixes #778

[j0shuams]

LGTM