fix: lower MSRV from 1.92.0 to 1.91.0

Edition 2024 is supported on Rust 1.85+, and 1.91.0 is the current
stable release. This allows cargo-rail to be installed on projects
that pin to Rust 1.91.x (like Meilisearch).

Author: loadingalias

Cargo.toml

@@ -2,7 +2,7 @@
 name = "cargo-rail"
 version = "0.5.2"
 edition = "2024"
-rust-version = "1.92.0"
+rust-version = "1.91.0"
 license = "MIT"
 authors = ["loadingalias"]
 description = "Graph-aware testing, dependency unification, and crate extraction for Rust monorepos"