Switch to using slnx instead of sln

[jakejscott]

What was changed

• Upgraded to use dotnet 9.0
• Upgraded solution to use slnx file format
• Temporal nuget packages updated to 1.13.0
• Updated coverlet.collector to 8.0.1 because a warning was shown that it had a vulnerability. If this package is not actually used it probably could be removed.

Why?

Checklist

1. Closes [Feature Request] Switch to using slnx instead of sln #132

2. How was this tested:

• Ran dotnet format--verify-no-changes
• Ran dotnet test

3. Any docs updates needed?

• Readme was updated so that users know they need dotnet 9.

[jmaeagle99]

Hi @jakejscott, thank you for your contribution. While I agree it would be great to update the solution file to use the slnx format, my understanding is that it is only support in the .NET 9+ SDKs. Since .NET 8 is still actively supported, I think it's best to keep the solution in a format that is compatible with all .NET 8+ SDKs for the time being.