Originally, `Base64Utils` was providing base64 encoding and decoding
utilities, bridging to commons-codecs or Java 8, if available. Since
then, only the Java 8 variant remains and Spring Framework 6 requires
now Java 17. This utility class doesn't provide additional checks or
syntactic sugar over what's in Java already.
As a result, this commit deprecates this class in favor of `Base64` and
schedules the removal of this class completely.
Closes gh-28434
5448e81021