Upgrade projects from .NET 6 to .NET 8 #14368
Description
Summary
We are still using .NET 6 for our codegen projects and requiring it in our dependency script. We should change that to .NET 8.
Motivation
.NET 6 is no longer supported. and will be a security/compliance issue.
Basic Example
No response
Open Questions
If we update the dependency script to only require .NET 8, then we need to make sure users setting up new projects for RNW versions that used .NET 6 will still work. We'd need to test on clean machines without .NET 6 to see that the Codegen targeting .NET 6 still works if only .NET 8 is installed.