lint: use as_chunks instead of chunks_exact

[jgreeer]

From a rust-clippy 1.98 CI run:

error: using `chunks_exact` with a constant chunk size
   --> rcgen/src/string.rs:430:8
    |
430 |             vec.chunks_exact(2)
    |                 ^^^^^^^^^^^^^^^ help: consider using `as_chunks` instead: `as_chunks::<2>().0.iter()`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.98.0/index.html#chunks_exact_to_as_chunks
    = note: `-D clippy::chunks-exact-to-as-chunks` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::chunks_exact_to_as_chunks)]`

error: using `chunks_exact` with a constant chunk size
   --> rcgen/src/string.rs:549:5
    |
549 |             .chunks_exact(4)
    |              ^^^^^^^^^^^^^^^ help: consider using `as_chunks` instead: `as_chunks::<4>().0.iter()`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.98.0/index.html#chunks_exact_to_as_chunks

    Checking regex-syntax v0.8.10
error: could not compile `rcgen` (lib) due to 2 previous errors

as_chunks stablized in rust 1.88.0, which is this crate's MSRV.

[djc]

Thanks!