Correct affected versions for checkin actions known issue (#15406)

The known-issues.md entry added in #15398 listed only 9.4.0-9.4.3, based
on an unverified assumption (pasted from a support case) that the
regression was 9.4.0-only. It was not: PR #14706, which changed
CheckinResponse.Actions from a typed slice to json.RawMessage, was
backported to every active branch on the same day. Lists only the
already-released affected patch versions on each line: 8.19.17-8.19.18,
9.3.6-9.3.7, and 9.4.3 (the fix ships today in the next patch on each
line, so no open-ended range is needed).

Relates #15397

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

Author: macdewee

docs/release-notes/known-issues.md

@@ -25,7 +25,7 @@ Known issues are significant defects or limitations that may impact your impleme
 
 :::{dropdown} {{agent}} logs a "failed to unmarshal checkin actions" error on almost every {{fleet}} check-in
 
-**Applies to: {{agent}} 9.4.0, 9.4.1, 9.4.2, 9.4.3**
+**Applies to: {{agent}} 8.19.17, 8.19.18, 9.3.6, 9.3.7, 9.4.3**
 
 On July 7, 2026, a known issue was discovered where {{fleet}}-managed {{agents}} log an error on nearly every check-in when there is nothing new for {{fleet}} to tell them to do: