Improve handling multiple delimiters in input

[sbulat]

In the recent update, bigdecimal now parses 1. inputs as 1, instead of throwing an exception (https://github.com/ruby/bigdecimal/pull/302). With the change, monetize parses inputs like 2..30 as 2 now, but previously it would throw an exception, passed down from BigDecimal. The spec is already in place: spec/monetize_spec.rb:406, but it won't fail until Ruby 3.4.0

This PR updates the code to preserve the behavior with the newest version of BigDecimal.

Additionally, from Ruby 3.4, BigDecimal is no longer part of the standard library, so this PR updates the Gemfile accordingly.

[sunny]

Can you add an entry to the changelog? 🙏🏻

[sbulat]

Thanks, updated!