maint: Drop runtime dependency on base64

[Earlopain]

1658 added base64 as a dependency in order to supress a warning/error on Ruby 3.3/3.4.

There are two places here where it is used, and these can instead be replaced by the pack method which the base64 gem just wraps around.

I've instead added it as a development dependency so that tests can continue to make use of it.

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.

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Before the change?

base64 is part of the depenency tree.

After the change?

base64 is removed.

Pull request checklist

• [ ] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• [ ] Yes
• [x] No

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