Status & README update, CI bumps for Ruby versions

[thbar]

In this PR:

• I remove Ruby 3.1 from the test matrix (it is EOL, and Minitest 6+ is not compatible with it anymore, for that reason as well)
• I add Ruby 3.4 & 4.0 (see Add Ruby 3.4 to testing matrix #105)
• Development dependencies have been updated as well
• I make an explicit mention of the current status of maintenance of Kiba (it is considered done ; I use it daily in production ; it will not see significant changes)
• Compatibility with Rubies has been updated in the README
• Work-around for CSV.open in write mode fails with "IOError: not opened for reading" when trying to strip BOM truffleruby/truffleruby#4081

Happy new year everyone 🥳