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:

• Remove base64 dependency rack/rack#2110
• Remove runtime dependency on base64 lostisland/faraday#1541
• [Fix #12310] Drop base64 gem from runtime dependency rubocop/rubocop#12313
• Remove dependency on base64 gem sidekiq/sidekiq#6151
• etc

[petergoldstein]

I like this approach.

[petergoldstein]

Thanks @Earlopain