Fix compilation on C# 11 (`scoped` refs) & migrate to .NET 7.0

[ReubenBond]

C# 11 introduces the scoped keyword which can be used in conjunction with ref in a method signature to specify that a reference does not escape the method.

I updated the GitHub Actions yamls to allow us to use prerelease versions of the .NET SDK and updated global.json to include the latest .NET 7.0 RC. We would likely want to revert the CI change after release.

Microsoft Reviewers: Open in CodeFlow