Fixes ruby deprecation warning regarding `bigdecimal` will be moved out from stdlib soon

[simonneutert]

What/Why

When using this gem in a project a warning is thrown.

/usr/local/bundle/gems/money-6.16.0/lib/money.rb:1: 
  warning: 
    bigdecimal was loaded from the standard library,  
    but will no longer be part of the default gems since  
    Ruby 3.4.0. Add bigdecimal to your Gemfile or gemspec.  
    Also contact author of money-6.16.0 to add bigdecimal  
    into its gemspec.

Shopify has a related issue to the topic, see their issue #1772.

What was done

• [x] fixed the specs, as one was erroring out. This could potentially break how CHF (Swiss Franks) were supposed to work. Please see https://github.com/RubyMoney/money/pull/1055 and provide some feedback 🙏
• [x] add bigdecimal gem to gemspec file with a condition for the used version of the gem
• [x] entry in CHANGELOG.md and AUTHORS