Put MSRV into Cargo.toml

Also remove it from clippy.toml as clippy
reads it from Cargo.toml since 1.64.
CI only runs clippy from latest stable and not
from the MSRV so this shouldn't be a problem.

Author: est31

Cargo.toml

@@ -13,6 +13,7 @@ A lightweight logging facade for Rust
 categories = ["development-tools::debugging"]
 keywords = ["logging"]
 exclude = ["rfcs/**/*"]
+rust-version = "1.60.0"
 
 [package.metadata.docs.rs]
 features = ["std", "serde", "kv_unstable_std", "kv_unstable_sval", "kv_unstable_serde"]