Add rust-version to Cargo.toml

[itsfarseen]

Problem:

• Cargo for Rust 1.59 is trying to pull bumpalo v3.12.1, which is doesn't compile on Rust 1.59.

Context:

• solana-program 1.10.x depends on bumpalo.
• Solana's BPF compiler still is on Rust 1.59

How to solve:

• Add rust-version = 1.60.0 to Cargo.toml

[itsfarseen]

This feature was added in Rust 1.56

See: https://github.com/rust-lang/cargo/blob/master/CHANGELOG.md#cargo-156-2021-10-21