Add release note ignore for backport note

This adds a directive to ignore a release note that we only want to show
up in a stable branch.

It also cleans up a previous instance of this where we needed to have
the ignore follow the release after stable branch creation.

Change-Id: I484bcb8aaa78548ddf2ef9063f689d533b1c692c
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

releasenotes/source/stein.rst

@@ -4,3 +4,5 @@
 
 .. release-notes::
    :branch: stable/stein
+   :ignore-notes:
+     multiple_clone-82bd7f80ae439080.yaml

releasenotes/source/unreleased.rst

@@ -4,4 +4,4 @@
 
 .. release-notes::
    :ignore-notes:
-     multiple_clone-82bd7f80ae439080.yaml
+     msa-multiattach-5407eb60093de8f1.yaml