Remove the default padding for RSA keyrings

[seebees]

The default was added when Node.js only supported OAEP-SHA-1-MFG1-SHA1,
but in v12 all the SHA flavors were added.

Having a default of SHA-1 when SHA-256 is available is a bad customer experience.
Since these values must agree on the keyring used to encrypt and the keyring to decrypt,
and this value will advance as the world changes,
having a default for this is a bad idea.

[seebees]

Also, all examples should be updated to SHA-256