Mention that stale issues will be closed.

This phrasing is in stale-pr-message, but missing from stale-issue-message.  This is probably because the source of 76305620a1, which was https://github.com/carbon-language/carbon-lang/blob/trunk/.github/workflows/stale.yaml, doesn't close issues (closing issues was added for protobuf's janitor).

PiperOrigin-RevId: 652956526

.github/workflows/janitor.yml

@@ -45,7 +45,8 @@ jobs:
 
 
             This issue is labeled `inactive` because the last activity was over
-            90 days ago.
+            90 days ago. This issue will be closed and archived after 14
+            additional days without activity.
           close-issue-message: >
             We triage inactive PRs and issues in order to make it easier to find
             active work. If this issue should remain active or becomes active