Update to recent dependencies

[undees]

A fresh clone of the NeverBounceApi-Ruby encounters a few errors trying to install gems and run the specs:

• The older Bundler version referenced in the Gemfile.lock raises errors trying to run it
• Starting with Ruby 3.1, the bigdecimal and csv gems are no longer included in the standard set of gems that ships with Ruby
• Starting with Ruby 3.4, same goes for the base64 gem
• The older version of json in the Gemfile.lock isn't compatible with newer Rubies
• One test case fails because newer Rubies use a single quote (') instead of a backtick (`) in a specific error message

This PR broadens the list of Ruby versions supported by this PR by doing the following:

• Moving all dependencies into the project's .gemspec
• Adding bigdecimal, csv, and base64 as dependencies, but only for the Ruby versions that need them
• Removing Gemfile.lock from revision control, because there wasn't a single BUNDLED WITH setting that would support all Rubies since 2.4 (and because Bundler recommends that gem repos omit the lock file)
• Adding a GitHub Action that runs the project's specs across all the major Ruby versions from 2.5 (the oldest version supported by the gem) to 3.4