Initial addition of .NET 8 support

[rhysparry]

Background

To ensure compatibility of Halibut in .NET 8 environments.

Results

• .NET 8 SDk
• Adds the net8.0 target
• Suppresses warnings associated with .NET 8
• [SC-85314]
• Resolves #615

How to review this PR

Quality :heavy_check_mark:

Pre-requisites

• [ ] I have read How we use GitHub Issues for help deciding when and where it's appropriate to make an issue.
• [ ] I have considered informing or consulting the right people, according to the ownership map.
• [ ] I have considered appropriate testing for my change.