Add 2.1.x backport label

Signed-off-by: Max Jonas Werner <mail@makk.es>

.github/labels.yaml

@@ -47,3 +47,6 @@
 - name: backport:release/v2.0.x
   description: To be backported to release/v2.0.x
   color: '#ffd700'
+- name: backport:release/v2.1.x
+  description: To be backported to release/v2.1.x
+  color: '#ffd700'