Upgrade Existing Contracts to ckb std 0.13 and Stable Rust

• Change rust-toolchain to use a stable version, e.g. 1.69.0
• Update ckb-std to 0.13 in contracts Cargo.toml files.
• Remove #![feature]s in contracts main.rs files.

It might be necessary to make some other code changes, see ckb-std changelog.