Also declare MSRV in Cargo.toml

[glts]

Follow-up to #1813: Also declare the MSRV in Cargo.toml, for the benefit of dependent apps and libraries.

See https://doc.rust-lang.org/cargo/reference/manifest.html#the-rust-version-field.

Thank you.

[djc]

LGTM! I guess CI is failing because of the new clippy lints in 1.65, would you be willing to clean those up too?

[glts]

I don’t really feel qualified to judge these lints (they seem somewhat subjective to me in this case).

If someone else wants to push commits here, feel free

[bluejekyll]

lgtm, I think we can merge this. I'll take care of the cleanliness in a separate PR in a minute.