Log Message:
ruby-kramdown: update to 2.3.1.

This release brings various bug fixes, one of which fixes a security issue
that can potentially lead to remote code execution.

2 minor changes:

- Added rel="footnote" to footnote links
- Default options are now cached to avoid unnecessary computations

6 bug fixes:

- Fixed abbreviations handling so that they won't be replaced in raw HTML tags
- Restricted Rouge formatters to Rouge::Formatters namespace
- Fixed conversion of empty raw html td/th tags to kramdown
- Fixed LaTeX entity conversion
- Fixed missing block attributes from IAL on standalone images
- Fixed inconsistency in how empty code spans are handle

4 other changes:

- Test on Ruby 2.7 and Ruby 3.0
- Fix rouge test failures due to new syntax
- Clarified requirements for rouge option formatter
- Removed warning in setup.rb