Update commonmarker gem to 0.23.9 (from 0.23.6)

What does this MR do and why?

Update to use the latest commonmarker gem, 0.23.9.

• https://my.diffend.io/gems/commonmarker/0.23.8/0.23.9
• https://my.diffend.io/gems/commonmarker/0.23.7/0.23.8
• https://my.diffend.io/gems/commonmarker/0.23.6/0.23.7

There are a couple changes in the output that are expected and acceptable, which are reflected in the glfm_specification/output_example_snapshots/html.yml, spec/lib/banzai/filter/footnote_filter_spec.rb, and spec/lib/banzai/pipeline/full_pipeline_spec.rb

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #412472 (closed)