Fix caching_sha2_password padding

[bgrainger]

From https://insidemysql.com/preparing-your-community-connector-for-mysql-8-part-2-sha256/:

It’s important to note that a incompatible change happened in server 8.0.5. Prior to server 8.0.5 the encryption was done using RSA_PKCS1_PADDING. With 8.0.5 it is done with RSA_PKCS1_OAEP_PADDING. This means that if you have implemented support for this authentication scheme for servers prior to 8.0.5 you will need to update your connector to make this change.