`rsa-export` is unmaintained

This crate has been deprecated in favour of using the native support for exporting RSA keys into the standard PEM format.
See [docs.rs documentation].

In addition to that, the operations in this crate (arithmetic and Base64 encoding) are not done in constant-time,
potentially [exposing the user to sidechannel attacks].

[docs.rs documentation]: https://docs.rs/rsa/0.9.6/rsa/index.html#pkcs8-rsa-key-encoding
[exposing the user to sidechannel attacks]: https://arxiv.org/pdf/2108.04600.pdf