Update .backportrc.json (#536)

Add 8.11 to backport tool branch list.

Author: kilfoyle

.backportrc.json

@@ -1,7 +1,7 @@
 {
   "repoOwner": "elastic",
   "repoName": "ingest-docs",
-  "targetBranchChoices": ["main", "8.10", "8.9", "8.8", "8.7", "8.6", "7.17"],
+  "targetBranchChoices": ["main", "8.11", "8.10", "8.9", "8.8", "8.7", "8.6", "7.17"],
   "autoMerge": true,
   "autoMergeMethod": "squash",
   "branchLabelMapping": {