Make es_release_notes choose the same :Label every time when multiple specified

dev-tools/es_release_notes.pl

@@ -161,7 +161,7 @@ ISSUE:
         # uncomment for including/excluding PRs already issued in other versions
         # next if grep {$_->{name}=~/^v2/} @{$issue->{labels}};
         my %labels = map { $_->{name} => 1 } @{ $issue->{labels} };
-        my ($header) = map { substr( $_, 1 ) } grep {/^:/} keys %labels;
+        my ($header) = map { substr( $_, 1 ) } grep {/^:/} sort keys %labels;
         $header ||= 'NOT CLASSIFIED';
         for (@Groups) {
             if ( $labels{$_} ) {