Migrate from `sun.misc.BASE64Decoder` and `sun.misc.BASE64Encoder` to `java.util.Base64`

[yeikel]

See https://stackoverflow.com/questions/13109588/encoding-as-base64-in-java

[pway99]

Thanks @yeikel, this will probably be similar to
ApacheBase64ToJavaBase64 in rewrite-migrate-java