maint: Added base64 gem explicitly dependency

[hsbt]

Before the change?

octokit.rb warns the following message With Ruby 3.3.0.

/Users/hsbt/.local/share/gem/gems/octokit-8.0.0/lib/octokit/client/code_scanning.rb:3: warning: base64 was loaded from the standard library, but is not part of the default gems since Ruby 3.4.0. Add base64 to your Gemfile or gemspec. Also contact author of octokit-8.0.0 to add base64 into its gemspec.

This means octokit.rb is not working after Ruby 3.4 release. We should add base64 as dependency.

After the change?

Above warning is suppressed.

Pull request checklist

• [x] 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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[github-actions[bot]]

👋 Hi! Thank you for this contribution! Just to let you know, our GitHub SDK team does a round of issue and PR reviews twice a week, every Monday and Friday! We have a process in place for prioritizing and responding to your input. Because you are a part of this community please feel free to comment, add to, or pick up any issues/PRs that are labled with Status: Up for grabs. You & others like you are the reason all of this works! So thank you & happy coding! 🚀

[hsbt]

@nickfloyd done to sort alphabetical order.

[hsbt]

Thanks!