doc: update changelog with 9.x EOL

This removes unsupported versions from the table, as those might confuse
people, execially given the
  > Release versions displayed in **bold** text represent the most
  > recent actively supported release.
remark below.

It was inconsistent to keep some of the EOL entries in the table while
removing others (v5.x, v7.x) -- this commit takes care of that.

Instead, release status is hinted in the branch list above, highlighting
two main branches -- Current and Active LTS.

Also update the link to the Release repo.

Refs: https://github.com/nodejs/Release/pull/351
PR-URL: https://github.com/nodejs/node/pull/21612


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>