backporting: Update labels by default when submitting backport

When submitting the backport PR using submit-backport, the script
proposes to update the labels (i.e., remove needs-backport/X and add
backport-pending/X):

    Sending pull request...
    Everything up-to-date
    cilium#13700

    Updating labels for PRs 13383 13608 12975

    Set labels for all PRs above? [y/N] y
    Setting labels for PR 13383... ✓
    Setting labels for PR 13608... ✓
    Setting labels for PR 12975... ✓

The choice defaults to not updating the labels. That may give the wrong
impression that it is an optional step---and if you're like me, when
you're unsure what an optional step does, you skip it. We should default
to setting the labels because we later rely on the labels being set
(e.g., when we update them after the PR is merged).

This commit changes it to the following:

    Sending pull request...
    Everything up-to-date
    cilium#13700

    Updating labels for PRs 13383 13608 12975

    Set labels for all PRs above? [Y/n]
    Setting labels for PR 13383... ✓
    Setting labels for PR 13608... ✓
    Setting labels for PR 12975... ✓

Signed-off-by: Paul Chaignon <paul@cilium.io>

contrib/backporting/submit-backport

@@ -51,9 +51,9 @@ hub pull-request -b "v$BRANCH" -l kind/backports,backport/$BRANCH -F $SUMMARY
 
 prs=$(grep "contrib/backporting/set-labels.py" $SUMMARY | sed -e 's/^.*for pr in \([0-9 ]\+\);.*$/\1/g')
 echo -e "\nUpdating labels for PRs $prs\n" 2>&1
-echo -n "Set labels for all PRs above? [y/N] "
+echo -n "Set labels for all PRs above? [Y/n] "
 read set_all_labels
-if [ "$set_all_labels" = "y" ]; then
+if [ "$set_all_labels" != "n" ]; then
     for pr in $prs; do
         $DIR/set-labels.py $pr pending $BRANCH;
     done