Update our MSRV to ≥ 1.62

It looks like if we want to use typetag (as we do for !1092 (merged)), we'll need Rust 1.62 or later.

Our policy is:

When increasing this MSRV, we won't require any Rust version released in the last six months. (That is, we'll only require Rust versions released at least six months ago.)

We will not increase MSRV on PATCH releases, though our dependencies might.

We won't increase MSRV just because we can: we'll only do so when we have a reason. (We don't guarantee that you'll agree with our reasoning; only that it will exist.)

Today is 2023-04-11; six months ago is 2022-10-11. So by the "six months" rule, we can set our MSRV as far as Rust 1.64 (released 2022-09-22). If we consider our rule to apply to release dates, and if we plan to release in early May, we could require Rust 1.65¹ (released 2022-11-03).

1. That's the one that stabilized GATs. Everybody wants GATs, right? 😕 ↩