Updated deprecated references in iOS 15.

[Jianhaos]

This is to fix all the warnings about the deprecated references.

This PR contains breaking changes, but I think they're worth keeping because:

1. We want to eliminate the warnings about the deprecated references.
2. The update requires changes in the public SwiftyRSAError enum which is inevitable IMO.
3. I believe some algorithms, like MD2 and MD5, are no longer supported so we will have to keep the old functions around to make them work because there is no one-to-one mapping, which feels cumbersome.