Fix graceful shutdown release note section

The graceful shutdown release note introduced by
I363c051ce112a28558214964ce276e47fe521e6c uses the unsupported
``known_issues`` Reno section. As a result, Reno does not publish the
migration task tracking issue in the generated release notes.

Use Reno's supported ``issues`` section so the known issue is rendered.
This release-note correction should be backported to branches that
contain the graceful shutdown feature.

Related-Bug: #2166009

Generated-By: OpenCode GPT-5.6 Terra
Signed-off-by: Sylvain Bauza <sbauza@redhat.com>
Change-Id: Icaae9a9cb9ed566aaad51e457d148f3c03255313

releasenotes/notes/nova-graceful-shutdown-task-tracking-13485c8346512ae4.yaml

@@ -8,7 +8,7 @@ features:
     only until they complete (or until
     ``[DEFAULT]/manager_shutdown_timeout`` is reached), instead of
     always sleeping for the full timeout.
-known_issues:
+issues:
   - |
     There is a known issue where a migration-related task tracked on the
     destination compute service may be left in-progress if an error on