Auto merge of #143214 - camsteffen:remove-let-chains-feature, r=est31

Remove let_chains unstable feature

Per rust-lang/rust#53667 (comment) (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.

Author: bors