Specify minimum rust version

[benmoss]

1.63 is required by Mutex::new added in #1442

Sorta fixes #1495 ? Doesn't really fix it, but makes it more clear to users that they're not gonna have a good time :)

Example of the error Cargo gives:

error: package just v1.11.0 (/workspace) cannot be built because it requires rustc 1.63 or newer, while the currently active rustc version is 1.62.1

It's not super clear at first glance but rust-version is just a minimum:

If the currently selected version of the Rust compiler is older than the stated version, cargo will exit with an error, telling the user what version is required.

[benmoss]

Maybe can do something fancy with https://crates.io/crates/cargo-msrv to do something in CI to keep this in sync

[casey]

I think that cargo-msrv is just too slow to run on CI. I think keeping the MSRV in sync manually is probably the best option.