chore: update msrv to 1.66.1

This Rust version is required due to the use of checked arithmetic
functions in locator.rs, otherwise the msrv would be 1.65 due to
dependencies.

Author: jpgrayson

Cargo.toml

@@ -3,7 +3,7 @@ name = "stgit"
 version = "2.3.0"
 description = "Stack-based patch management for Git"
 edition = "2021"
-rust-version = "1.64"
+rust-version = "1.66.1"
 authors = [
   "Catalin Marinas <[email protected]>",
   "Peter Grayson <[email protected]>",