api: require base64

This was required previously by the `plist` gem and we were requiring
it indirectly. It got removed in v1.7.1 of that gem because it will
no longer be included in the standard library as of Ruby 3.4.

Since we're not currently running on Ruby 3.3 and 3.4 is not out yet,
we can punt on this for now but we'll need to figure out a better
alternative before upgrading to Ruby 3.4.

Related:
- patsplat/plist#63

Library/Homebrew/api.rb

@@ -4,6 +4,7 @@
 require "api/analytics"
 require "api/cask"
 require "api/formula"
+require "base64" # TODO: Add this to the Gemfile or remove it before moving to Ruby 3.4.
 require "extend/cachable"
 
 module Homebrew