Drop depenency on base64

[Earlopain]

#759 added base64 as a depenency in order to suppress a warning/error on Ruby 3.3/3.4

Since base64 is mostly just a wrapper around the pack/unpack methods it's easy to just replace.

Other gems have taken a similar approach:

• https://github.com/rubocop/rubocop/pull/12313
• https://github.com/rack/rack/pull/2110
• https://github.com/elastic/elasticsearch-ruby/pull/2295
• https://github.com/socketry/protocol-http/pull/51
• etc.