Explicit sub-steps for assessing -S and EOL

Explicit sub-steps for assessing -S and EOL

For the step where we assess which product versions/branches are
vulnerable to the flaw, add explicit subordinate steps for assessing
Special Subscriber -S Preview edition, and end-of-life versions that
are still received paid fixes.

While we have GitLab labels to indicate affected versions, there is no
satisfactory mechanism in place to indicate that assessment of all
versions is complete, and thus anything not labeled as affected can be
considered immune.  Explicit checklist steps will allow others to see
when assessment is complete.

Per the following discussions:

https://zulip.isc.org/#narrow/channel/4-bind9/topic/Unaffected.20labels.20for.20vulnerability.20issues/near/25643

https://zulip.isc.org/#narrow/channel/4-bind9/topic/CVE.20checklist.20updates/near/26307