Wrong Description in Base-64/Radix-64 Method

When encoding/decoding using base64-method, multiple alphabets are available. In addition to the character-set, a description is provided. There is a alphabet called `Radix-64 (RFC 4880)` defined in `src/core/lib/Base64.mjs`. The RFC in parentheses was introduced by [4cf80e3ebbda68091150ac6f337d950223bcd2ec](https://github.com/gchq/CyberChef/commit/4cf80e3ebbda68091150ac6f337d950223bcd2ec). The alphabet in question is `0-9A-Za-z+/=`.

However, the alphabet described in [RFC 4880](https://datatracker.ietf.org/doc/html/rfc4880#section-6.3) called `Radix-64` is not different from the normal Basse-64 alphabet. I could not find an RFC/… for this alphabet called `Radix-64`, but found usage in the wild, but called `Base-64`.

I suggest to remove the `RFC 4880` in the description, leave it as `Radix-64` and if possible, refer to the correct standard.

Start in src/core/lib/Base64.mjs at the Radix-64 (RFC 4880) alphabet definition, then compare its description with the linked RFC 4880 section. Remove the RFC reference if it is not applicable and confirm the user-facing alphabet description is accurate.

Written by the indexing model from the issue text.