Suppress a Ruby warning when using Ruby 3.3.0dev

Follow up rubocop#12313.

`base64` is not needed at runtime, but it was required as a test dependency.
So, this PR suppresses the following warning when using Ruby 3.3.0dev:

```consle
$ ruby -v
ruby 3.3.0dev (2023-10-23T08:04:27Z master e6fcf07a6f) [x86_64-darwin22]

$ cd path/to/rubocop
$ bundle exec rspec
/Users/koic/.rbenv/versions/3.3.0-dev/lib/ruby/gems/3.3.0+0/gems/webmock-3.19.1/lib/webmock/util/headers.rb:3:
warning: base64 which will no longer be part of the default gems since Ruby 3.4.0. Add base64 to your Gemfile or gemspec.
```

https://app.circleci.com/pipelines/github/rubocop/rubocop/10108/workflows/20097132-bbe3-4fa4-ad7e-de7c30c86e69/jobs/291956?invite=true#step-104-0_163

The `base64` dependency can be removed from Gemfile when bblimke/webmock#1041 is merged and released.
It's a workaround until then.

Gemfile

@@ -22,6 +22,9 @@ gem 'test-queue'
 gem 'yard', '~> 0.9'
 
 group :test do
+  # FIXME: This `base64` dependency can be removed when https://github.com/bblimke/webmock/pull/1041
+  # is merged and released. It's a workaround until then.
+  gem 'base64'
   # FIXME: This `bigdecimal` dependency can be removed when https://github.com/jnunemaker/crack/pull/75
   # is merged and released. It's a workaround until then.
   gem 'bigdecimal', platform: :mri