chore(deps): Document current MSRV in Cargo.toml

This is the current defacto MSRV not an upgrade because the last release
with locked deps already requires this, it just wasn't specifically
documented. Documenting it should help in the future analizing what
release tags should be and whether or not to dump dependencies.

The current limitation comes via the `anstyle` and `clap_lex`
dependencies burried in the dependency tree.

Author: alerque

Cargo.toml

@@ -3,6 +3,7 @@ name = "git-warp-time"
 version = "0.5.1"
 authors = ["Caleb Maclennan <[email protected]>"]
 edition = "2021"
+rust-version = "1.70.0"
 description = "Rust library and CLI utility to reset file timestamps to repo state"
 readme = "README.md"
 homepage = "https://github.com/alerque/git-warp-time"