Add 1.6.x release label

Signed-off-by: Matheus Pimenta <matheuscscp@gmail.com>

Author: matheuscscp

.github/labels.yaml

@@ -25,3 +25,6 @@
 - name: backport:release/v1.5.x
   description: To be backported to release/v1.5.x
   color: '#ffd700'
+- name: backport:release/v1.6.x
+  description: To be backported to release/v1.6.x
+  color: '#ffd700'