Enforce MSRV in CI

This crate advertises a 1.70 MSRV but generates this error: ```rust error[E0658]: mutable references are not allowed in constant functions --> src/bit_vector.rs:28:22 | 28 | pub const fn set(&mut self, index: usize, hash: u64) -> bool { | ^^^^^^^^^ | = note: see issue #57349 <https://github.com/rust-lang/rust/issues/57349> for more information ``` This only works from 1.82 on. In Quinn, we prefer to be more conservative about the MSRV (currently at 1.71, though we're bumping to 1.74 soon) so if there's a way to decrease it again, that would be nice.