doc: add 9.x to version picker and mark 8.x as LTS
Created by: chris--young
Fixes: https://github.com/nodejs/node/issues/16671
Checklist
-
make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines
Affected core subsystem(s)
doc, tests