This commit updates the optional rustls-ffi librustls dependency from
0.12.0 to 0.13.0. This version is based on the latest available rustls
release (0.23.4).
The breaking API changes from 0.12.0 to 0.13.0 are in API surface unused
by curl, so this is an in-place update without any code changes.
The `RUSTLS.md` documentation is updated to reflect the new version in
use, and to clarify that `cbindgen` isn't required to build `librustls`
- it's only used by developers to update the vendored `rustls.h` header
file maintained upstream.
Closes#13238
5b1a88e2c0