rusts-pemfile has been archived and won't see future updates. The
rustls-pki-types crate has supported PEM parsing since 1.9.0.

This commit converts the existing parsing while maintaining semver
compatibility, with the exception of the Error::Pem() enum variant,
that exposed a rustls-pemfile type. This breaking change is **not**
considered in-scope for ureq's semver policy based on the pre-existing
rustdoc comment.