Remove let_chains unstable feature Per https://github.com/rust-lang/rust/issues/53667#issuecomment-3016742982 (but then I also noticed rust-lang/rust#140722) This replaces the feature gate with a parser error that says let chains require 2024. A lot of tests were using the unstable feature. I either added edition:2024 to the test or split out the parts that require 2024.