Identify/decide minimum supported version of Rust

C.f. #1238 If I understand correctly: https://github.com/rust-lang/git2-rs/blob/7415113423d3b852e3527a62a97a8505ad9a7fe5/Cargo.toml#L16 means that MSRV >= 1.56.0 https://github.com/rust-lang/git2-rs/blob/7415113423d3b852e3527a62a97a8505ad9a7fe5/Cargo.toml#L20 means MSRV >= 1.56.0 (https://docs.rs/crate/bitflags/2.1.0/source/Cargo.toml#55) https://github.com/rust-lang/git2-rs/blob/7415113423d3b852e3527a62a97a8505ad9a7fe5/Cargo.lock#L474-L476 means MSRV >= 1.63.0 (https://docs.rs/crate/libc/0.2.174/source/Cargo.toml#52) https://github.com/rust-lang/git2-rs/blob/7415113423d3b852e3527a62a97a8505ad9a7fe5/src/cred.rs#L301 means MSRV >= 1.70.0 (https://blog.rust-lang.org/2023/06/01/Rust-1.70.0/) (unless we want to say that there is a different MSRV depending on which features are enabled?) https://github.com/rust-lang/git2-rs/blob/7415113423d3b852e3527a62a97a8505ad9a7fe5/src/opts.rs#L527 means *tests* are MSRV >= 1.70.0 for the same reason