Upgrade to .NET 7, enable WASM optimizations

[Arlodotexe]

Companion PR to CommunityToolkit/Tooling-Windows-Submodule#117, which should be closed before merging this.

As noted:

Any codebase consuming the tooling repo will need to be adjusted to account for the new nullable annotations and analyzers added between netstandard2.0 and net7.0.

This PR pulls in the .NET 7 upgrade and fixes the new warnings and errors that appeared in our codebase as a result.

[michael-hawker]

@Arlodotexe can you see if any of the open PRs need updating from this? Thanks!

[michael-hawker]

Wasn't sure why GitHub wasn't reporting anything, had to dig-in to the Actions page for the last build and see the 'Startup Failure' seems like one of the actions isn't on the allowlist, going to fix that.

[michael-hawker]

Going to close and re-open to kick off CI again

[michael-hawker]

@Arlodotexe CI is running again, but something up with the call to the pack script:

   2 |  ./PackEachExperiment.ps1 -extraBuildProperties "pull-485.1577"
     |                           ~~~~~~~~~~~~~~~~~~~~~
     | A parameter cannot be found that matches parameter name 'extraBuildProperties'.

Maybe the sub-module isn't updated or something? Should hopefully be an easy fix of some sort either way.

[Jay-o-Way]

Hey guys, check out the MS video: "Upgrade your .NET projects with Visual Studio"
https://build.microsoft.com/en-US/sessions/c5bf6653-f0de-4091-a139-caf7904300c6

[michael-hawker]

@Arlodotexe this is still needed in between the added PR step and the 'Push packages' still. This is why main is failing, as the source isn't updated to include the token.