Drop dependency on base64

[Earlopain]

982 added base64 as a dependency in order to avoid a warning with Ruby 3.3

The base64 gem is mostly a wrapper around pack/unpack and as such is easy to replace. Other gems have opted for the same approach:

• https://github.com/rack/rack/pull/2110
• https://github.com/lostisland/faraday/pull/1541
• https://github.com/rubocop/rubocop/pull/12313
• https://github.com/sidekiq/sidekiq/pull/6151
• etc

[petergoldstein]

I like this approach.

[petergoldstein]

Thanks @Earlopain