Bump to MSRV 1.{60,61} for Arti 1.1.0?

In August, we decided not to bump our MSRV to 1.59 for Arti 1.0.0 (#538 (closed)).

By our rules, we could now bump to MSRV 1.60 for 1.1.0. In two weeks (November 19), we could bump to MSRV 1.61.

In addition to the reasons discussed in #538 (closed) for upgrading to ≥1.59, here are the reasons for updating to 1.60:

• Greatly improved cargo feature handling. We'd use this in #434 (closed).

For requiring 1.61, we would also get:

• Better const fns.
• A few stabilized APIs that we would use (including Vec::retain_mut).

These are all nice-to-have, but not IMO knock-down compelling. IMO the main reason to consider updating our MSRV is the number of our dependencies whose latest versions require a higher MSRV. They are currently:

• rsa (#613 (closed))
• tinystr (#591 (closed))
• serde_with, phf (#526 (closed))
• clap (!829 (closed))