Add missing labels to stalebot config

We missed a handful of labels in our stalebot config.
The presence of these labels should prevent closing.

Signed-off-by: Craig Jellick <craig@rancher.com>

.github/stale.yml

@@ -22,6 +22,12 @@ exemptLabels:
   - kind/ci-improvements
   - kind/performance
   - kind/flaky-test
+  - kind/documentation
+  - kind/backport
+  - priority/backlog
+  - priority/critical-urgent
+  - priority/important-longterm
+  - priority/important-soon
 
 # Set to true to ignore issues in a project (defaults to false)
 exemptProjects: true
@@ -46,4 +52,4 @@ markComment: >
 limitPerRun: 30
 
 # Limit to only `issues`
-only: issues
+only: issues