[docs] Add references to the 8.17 changelog in CHANGELOG.asciidoc

[colleenmcginnis]

Motivation/summary

In #14929, the 8.17 changelog was added to the main branch, but it is not referenced in the CHANGELOG.asciidoc file, so it's not being built in the docs.

This is blocking elastic/observability-docs#4667.

Checklist

• Update CHANGELOG.asciidoc
• Documentation has been updated

How to test these changes

View the docs preview after it's done building:

• Release notes should include a list item linking to the 8.17 changelog
• APM version 8.17 should exist

Related issues

#14929

[mergify]

This pull request does not have a backport label. Could you fix it @colleenmcginnis? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-7.17 is the label to automatically backport to the 7.17 branch.
• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• backport-8.x is the label to automatically backport to the 8.x branch.

[mergify]

backport-8.x has been added to help with the transition to the new branch 8.x.
If you don't need it please use backport-skip label.

[colleenmcginnis]

These changes already exist in 8.x: https://www.elastic.co/guide/en/observability/8.x/apm-release-notes.html. Removing the backport-8.x label.