rustls-pemfile is unmaintained

The rustls-pemfile crate is no longer maintained. The repository has been archived since August
2025, and users are encouraged to depend directly on the underlying PEM parsing code included
in rustls-pki-types since 1.9.0. The latest version of rustls-pemfile is in fact a thin wrapper
around the same code used in rustls-pki-types, so migrating should be straightforward.

The new API is represented by the [`PemObject`][PemObject] trait, which provides methods for
reading a single or multiple PEM objects from a file or byte slice.

[PemObject]: https://docs.rs/rustls-pki-types/latest/rustls_pki_types/pem/trait.PemObject.html