chore: Update dependencies and fix test suite

[evansims]

Changes

This PR updates the Gemfile dependencies to avoid installing potentially incompatible new majors, updates the dependencies lockfile to address a Snyk warning, and fixes a missing test dependency multi_json which was appearently removed from the upstream transient oauth2 dependency with its 2.0 release.

References

N/A

Testing

Please describe how this can be tested by reviewers. Be specific about anything not tested and reasons why. If this library has unit and/or integration testing, tests should be added for new functionality and existing tests should complete without errors.

• [ ] This change adds unit test coverage
• [x] This change has been tested on the latest version of the platform/language or why not

Checklist

• [x] I have read the Auth0 contribution guidelines
• [x] I have read the Auth0 Code of Conduct
• [x] All existing and new tests complete without errors
• [x] All code quality tools/guidelines in the CONTRIBUTING documentation have been run/followed

[evansims]

@stevehobbsdev I'm not familiar enough with Ruby or bundle to know if I went over the top with my restrictions on the Gemfile, but I noticed that bundle install by default wanted to install potentially incompatible new majors of dependencies like Sinatra. I thought I'd err on the side of caution and be verbose in the notation, but please let me know if I need to adjust the PR to be less so.