issue-6: update dependencies for ruby 3.x+; remove outdated ruby versions

[ac21]

resolves #6

• Updates for current ruby versions (2.7, 3.0, 3.1, 3.2)
• Adds required dependency on commonmarker
• provides future support for commonmarker 1+ and html-pipeline 3+

[ac21]

@winston - if you are not interested in supporting this repo going forward, we at codetree would be willing to help as we use this filter and others.

[ac21]

@winston - thoughts?