As suggested [on zulip][1], there's no need to use `GenericBound` here,
as the only bound a lifetime can have is that it outlives other
lifetimes.
While we're making breaking changes here, I also renamed it from using
"region" to "lifetime", as this is more user-aligned. See [this
comment][2] for details.
[1]: 448871430
[2]: https://github.com/rust-lang/rust/issues/100961#issuecomment-2206565556
