Fix a lint error that occurred when using Ruby 3.4

In Ruby 3.4, base64 is no longer part of the standard library, which was causing the following error.

```
Run bundle exec rake rubocop
  bundle exec rake rubocop
  shell: /usr/bin/bash -e {0}
rubocop -c .rubocop.yml --display-only-fail-level-offenses -D
/home/runner/work/ruby-http-client/ruby-http-client/vendor/bundle/ruby/3.4.0/gems/rubocop-1.24.1/lib/rubocop/formatter/html_formatter.rb:5: warning: ostruct was loaded from the standard library, but will no longer be part of the default gems starting from Ruby 3.5.0.
You can add ostruct to your Gemfile or gemspec to silence this warning.
/home/runner/work/ruby-http-client/ruby-http-client/vendor/bundle/ruby/3.4.0/gems/rubocop-1.24.1/lib/rubocop/formatter/html_formatter.rb:6: warning: base64 was loaded from the standard library, but is not part of the default gems starting from Ruby 3.4.0.
You can add base64 to your Gemfile or gemspec to silence this warning.
/opt/hostedtoolcache/Ruby/3.4.6/x64/lib/ruby/3.4.0/bundled_gems.rb:82:in 'Kernel.require': cannot load such file -- base64 (LoadError)
	from /opt/hostedtoolcache/Ruby/3.4.6/x64/lib/ruby/3.4.0/bundled_gems.rb:82:in 'block (2 levels) in Kernel#replace_require'
	from /home/runner/work/ruby-http-client/ruby-http-client/vendor/bundle/ruby/3.4.0/gems/rubocop-1.24.1/lib/rubocop/formatter/html_formatter.rb:6:in '<top (required)>'
	from /home/runner/work/ruby-http-client/ruby-http-client/vendor/bundle/ruby/3.4.0/gems/rubocop-1.24.1/lib/rubocop.rb:669:in 'Kernel#require_relative'
	from /home/runner/work/ruby-http-client/ruby-http-client/vendor/bundle/ruby/3.4.0/gems/rubocop-1.24.1/lib/rubocop.rb:669:in '<top (required)>'
	from /opt/hostedtoolcache/Ruby/3.4.6/x64/lib/ruby/3.4.0/bundled_gems.rb:82:in 'Kernel.require'
	from /opt/hostedtoolcache/Ruby/3.4.6/x64/lib/ruby/3.4.0/bundled_gems.rb:82:in 'block (2 levels) in Kernel#replace_require'
	from /home/runner/work/ruby-http-client/ruby-http-client/vendor/bundle/ruby/3.4.0/gems/rubocop-1.24.1/exe/rubocop:6:in '<top (required)>'
	from /home/runner/work/ruby-http-client/ruby-http-client/vendor/bundle/ruby/3.4.0/bin/rubocop:25:in 'Kernel#load'
	from /home/runner/work/ruby-http-client/ruby-http-client/vendor/bundle/ruby/3.4.0/bin/rubocop:25:in '<main>'
rake aborted!
Command failed with status (1): [rubocop -c .rubocop.yml --display-only-fail-level-offenses -D]
/home/runner/work/ruby-http-client/ruby-http-client/Rakefile:9:in 'block in <top (required)>'
/home/runner/work/ruby-http-client/ruby-http-client/vendor/bundle/ruby/3.4.0/gems/rake-13.3.0/exe/rake:27:in '<top (required)>'
/opt/hostedtoolcache/Ruby/3.4.6/x64/bin/bundle:25:in 'Kernel#load'
/opt/hostedtoolcache/Ruby/3.4.6/x64/bin/bundle:25:in '<main>'
```

Author: willnet

.github/workflows/test-and-deploy.yml

@@ -33,7 +33,9 @@ jobs:
       - run: make install
 
       - name: Set up linter
-        run: bundle add rubocop --version "~> 1.24.1" --group "development" --skip-install
+        run: |
+          bundle add rubocop --version "~> 1.24.1" --group "development" --skip-install
+          bundle add base64 --group "development" --skip-install
         if: ${{ matrix.ruby != '2.4' }}
 
       - run: bundle install --with development