Bump MSRV to 1.71

[ljoss17]

Update to Rust release 1.71.0

🦀

• Update the version in rust-toolchain.toml.
• Run cargo clippy --all-features --fix, review and commit the automatic
  fixes, and fix all reported lints. Try to resolve the root causes for
  the lints rather than silencing them with #[allow(...)].

Update the MSRV (optional)

Additional steps to perform in order to make the new minimal supported
Rust version:

• Update the rust-version fields in all Cargo.toml files.
• Update the msrv field in clippy.toml and fix all lints reported by
  cargo clippy --all-features.
• Update the MSRV shields in README files:
  • README.md
  • relayer-rest/README.md
• Update the MSRV in the guide: guide/src/pre_requisites.md
• Add a .changelog entry to the breaking-changes section,
  announcing the new MSRV.