Fix for "let chains are only allowed in Rust 2024 or later" errors when downgrading
Cargo.toml edition from 2024 to 2021. Use when: (1) changing edition = "2024" to
edition = "2021", (2) compilation fails with let chain errors across multiple files,
(3) need to systematically refactor `if let ... && let ...` patterns to nested if-let.
Covers the refactoring patterns for converting let chains to 2021-compatible code.