update MSRV to 1.52

[djmitche]

Description

This updates the minumum Rust version to 1.51, which is required by some updated dependencies. Since nothing depends on these Rust libraries yet, there's no reason not to bump it as necessary

[djmitche]

Hm, 1.51 still failed. I don't know a reliable way to determine stabilization versions for a lint. So, guess-and-check :)

[djmitche]

Yay! (Let's squash this when merging)

[dbr]

I don't know a reliable way to determine stabilization versions for a lint

The "note: see issue #71668 rust-lang/rust#71668 for more information" part usually links to a tracking issue, which usually has a link to the stablization issue, which in this case is
rust-lang/rust#79208
which is usually linked to a milestone for the relevant release (in this case 1.52)
..but as evidenced by using the word "usually" three times, isn't exactly reliable and the relevant stablization issues can be pretty hard to find sometimes

Haven't tried it but there is https://github.com/foresterre/cargo-msrv - which appears to basically be build-each-version-bisection-till-it-works