CI: add bigdecimal to Gemfile

[m-nakamura145]

From Ruby 3.4 onwards, gems that are expected to become bundled gems will raise a LoadError if they are required without being present in the Gemfile or gemspec. Therefore, I have added the target gem to the Gemfile.

/addressable/spec/addressable/template_spec.rb:20: warning: bigdecimal was loaded from the standard library, but is not part of the default gems since Ruby 3.4.0. Add bigdecimal to your Gemfile or gemspec.

An error occurred while loading ./spec/addressable/template_spec.rb. - Did you mean?
                    rspec ./spec/addressable/idna_spec.rb
                    rspec ./spec/addressable/uri_spec.rb
                    rspec ./spec/addressable/security_spec.rb

Failure/Error: require "bigdecimal"

LoadError:
  cannot load such file -- bigdecimal
# ./spec/addressable/template_spec.rb:20:in '<top (required)>'

[dentarg]

We don't need to add this preview version, we have a job for head, which now is ruby 3.4.0dev (https://github.com/sporkmonger/addressable/actions/runs/9173838413/job/25223381402#step:5:25)

Last time that job ran was 4 months ago, so I suspect the bigdecimal change had not happen yet as it passed back then (but the logs are now gone)

[dentarg]

You need to make this work for the jobs that are failing