chore(stale.yml): Do not mark as stale PRs from dependabot

Signed-off-by: Lluis Campos <lluis.campos@northern.tech>

.github/stale.yml

@@ -15,3 +15,6 @@ markComment: >
   for your contributions.
 # Comment to post when closing a stale issue. Set to `false` to disable
 closeComment: false
+# Issues or Pull Requests with these labels will never be considered stale. Set to `[]` to disable
+exemptLabels:
+  - dependencies