Add base64 to gemspec

[shmokmt]

Description

• Add base64 to gemspec, which is no longer the default gem from Ruby 3.4.
  • opensearch-ruby uses base64 gem in client.rb

    ❯ git grep "require 'base64'"
    lib/opensearch/transport/client.rb:27:require 'base64'

References

• similar support in rails/rails: https://github.com/rails/rails/pull/48907
• bugs.ruby-lang: https://bugs.ruby-lang.org/issues/19776
• ruby/ruby: https://github.com/ruby/ruby/pull/8126

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