chore(msrv): stablize 'let' statements

[billf]

edition 2024 existed in rust versions previous to 1.88, but 'let' chaining syntax had not yet settled down.

used https://foresterre.github.io/cargo-msrv

This feature was stabilized in 1.88.0 but on edition 2024 only.

c.f. rust-lang/rust#53667

[billf]

firing off a release to address #139 would also be nice