[Doc]Remove old compatibility notes

[karenzone]

These notes don't make sense in 7.x (or 6.x for that matter). Documentation for older versions is still available if somebody needs it.

[yaauie]

• 1 to removing these from modern docs, since Logstash 2.x was EOL'd on 2018-02-28 with the release of 6.0.

These docs will continue to exist in the versioned-docs archive, so users who are still in this situation will be able to find them (e.g., we recommend updating through each major's last minor when doing multi-major upgrades, so someone who missed the boat for 5.x in 2016 and is just now planning a 2.x->7.x upgrade will still need this info).