Define required ruby version to >=2.7.0

[difernandez]

Motivation / Background

This Pull Request has been created because we are supporting ruby 2.6, which has been in EOL for a year. Having 2.6 in our test suite throws error when upgrading nokogiri dependency:

Detail

This Pull Request changes removes 2.6 from CI, and sets required_ruby_version in gemspec

Additional information

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a concise description of what changed and why.
• Tests are added or updated if you fix a bug or add a feature.
• Documentation has been added or updated if you add a feature or modify an existing one.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature (under the "Unreleased" heading if this is not a version change).
• My changes don't introduce any linter rule violations.