Add base64 dependency to gemspec

[UlyssesZh]

base64 is already removed starting from Ruby 3.4. We have to add this to gemspec. Otherwise, projects using Ruby 3.4 depending on liquid need to manually add base64 to their Gemfile.

Closes #1772. In that issue, @olegantonyan says he did not open a PR to add base64 to gemspec because there were no dependencies in gemspec at that time. Now there is bigdecimal and strscan dependency in gemspec, I think it is proper to add base64 too.

Related:

• https://github.com/Shopify/liquid/pull/1882

[UlyssesZh]

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