Update Language Level to C# 12

[CharliePoole]

Required by #1539

[manfred-brands]

Any reason not to go to C# 12 as per .NET 8.0?

[CharliePoole]

No, it's just the level I'm working at in my other project. I see the framework is using 12, so I'll do it too.

FYI, in doing this I discovered that the build, which hasn't run for many months, no longer works for V4. Due to EOL events and other stuff. I'm doing a very messy fix to get it working again. Long-lived branches that aren't being worked on should probably be built on a schedule.