Update Asciidoc build messaging to align with updated engineering maintenance policy

#### "With the release of 8.16, we are also enhancing our [Engineering Maintenance Policy](https://www.elastic.co/support/eol). Elastic will now maintain the most current minor version as well as the previous minor version. This change increases the time customers can remain on a recent minor version and continue receiving maintenance updates. This is huge for our customers."

- [ ] Priority 1: Remove the "no additional bug fixes or documentation updates" messaging for the previous minor. This messaging directly contradicts the new maintenance policy.


- [ ] Evaluate pros and cons of including the previous minor in the version dropdown. Consider SEO and other implications.


- [ ] Communicate to all Docs team members that we should be backporting to the previous minor in addition to current version.