Update rust to 1.71.1

[Manishearth]

Needed by pulldown_cmark v12, pulled in by the Diplomat upgrade (#5482). This is a diplomat-tool dep and not user-facing, but our CI is set up in a way that that trips our MSRV check. We could exclude the diplomat regen crate from that CI job.

Current Rust stable is 1.80.1, by policy:

One may update the MSRV to a Rust version that is 6 or more versions before the current stable (or the stable version that shall exist at the time of the next ICU4X release if it is planned) without needing additional approval by stating which features in that release are useful for ICU4X development. One may not update the MSRV to a Rust version that is not at least 4 versions before current stable.

We do not need further justification according to policy, though we don't actually really need any features from there either.