Add Cassandra to endoflife.date

A few Apache projects are featured on https://endoflife.date/ which is a community-sourced tracker of end-of-life dates and support timelines:

• zookeeper
• solr

Instructions on how to add a new project can be found on https://endoflife.date/contribute#new-adding-a-new-product .

Cassandra EOL info can be found on https://cassandra.apache.org/_/download.html, and possibly on dev mailing list threads.