[7.17] Add base64 as a dependency

[Earlopain]

It is my understanding that this branch is still maintained. You know the drill:

.../elasticsearch-ruby/elasticsearch-transport/lib/elasticsearch/transport/client.rb:18: warning: base64 was loaded from the standard library, but will no longer be part of the default gems since Ruby 3.4.0. Add base64 to your Gemfile or gemspec.

preview1 for 3.4 has been released so this is starting to become an actual issue.

Could also do the unpack/pack thing I did for 8.x but 🤷, this is less invasive (not that that change was invasive in the first place though)

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Also add CI for Ruby 3.3

[Earlopain]

CI failed on 3.3 for some reason: https://github.com/elastic/elasticsearch-ruby/actions/runs/9350312476/job/25733564008 Not sure, maybe it got loaded throuh some dependency beforhand or I just got unlucky with load order or something. I ran locally with 3.3 and that was fine

[picandocodigo]

Thanks again @Earlopain!