bump minimum supported rust version (MSRV)?

[ctb]

per #3012 (comment),

(asking here, because it's relevant to some memory questions) - how are we feeling about Rust 1.74 these days?

I've been using https://lib.rs/stats#rustc-usage to check MSRV bumps, 1.74 would put us in 96-97% of all crates.io requests. 1.74 was released November 16, 2023, so I think it is OK to bump MSRV.

The latest version of rkyv (see #3511) wants 1.81.

The latest version of niffler (see #3510) wants 1.74.

[luizirber]

Covered in #3530 . After it is merged we can cut r0.19.0, for now it was only bumped in latest

I avoided rkyv upgrades because 1.81 is a bit too new.

[luizirber]

Fixed in #3530, released in https://github.com/sourmash-bio/sourmash/releases/tag/r0.19.0