fix: downgrade to edition 2021 and MSRV 1.80 for stable Rust compatibility

Edition 2024 requires Rust 1.85+ which breaks installation for users
on stable toolchains (e.g., Rust 1.84). Since we support Python 3.9+
targeting enterprise environments, a conservative MSRV aligns with
that philosophy.

Changes:
- rust/Cargo.toml: edition 2024 → 2021, rust-version 1.91 → 1.80

Fixes #30

Author: 27Bslash6

rust/Cargo.toml

@@ -1,10 +1,10 @@
 [package]
 name = "cachekit-rs"
 version = "0.2.1"
-edition = "2024"
+edition = "2021"
 authors = ["cachekit Contributors"]
 description = "High-performance storage engine for caching with compression and encryption"
-rust-version = "1.91"
+rust-version = "1.80"
 license = "MIT"
 repository = "https://github.com/cachekit-io/cachekit-py"
 homepage = "https://github.com/cachekit-io/cachekit-py"