`2.0` release

[KodrAus]

I'm planning a 2.0 of emit that will make some very minor technically breaking changes to fix some mistakes and papercuts. I'll try collect a list of them here:

Breaking changes

• Update to 2024 edition #360 Update to the 2024 edition. This seems to affect lifetime handling in downstream crates too so isn't safe to do in a non-breaking way
• Remove deprecated APIs
• Movable props codegen #374 Make props!, evt!, metric!, and span_guard! capture by-value by default, leaving emit!, sample!, and #[span] capturing by-reference
• Reconsider handling of #[cfg] in templates #341: Make #[cfg] in holes still produce the hole, but with an uncaptured value
• Make Span and Metric pull well-known props from P instead of direct fields #376: Make Metric and Span only accept impl Props and pull well-known props from them instead of special-casing some

Note that emit_core will remain 1.0. There isn't any need to break anything there. emit 2.0 will use the same runtimes as 1.0.