Remove 8.5 branch from backport config

.backportrc.json

@@ -1,9 +1,9 @@
 {
   "upstream" : "elastic/elasticsearch",
-  "targetBranchChoices" : [ "main", "8.5", "8.4", "8.3", "8.2", "8.1", "8.0", "7.17", "6.8" ],
+  "targetBranchChoices" : [ "main", "8.4", "8.3", "8.2", "8.1", "8.0", "7.17", "6.8" ],
   "targetPRLabels" : [ "backport" ],
   "branchLabelMapping" : {
     "^v(\\d+).(\\d+).\\d+(?:-(?:alpha|beta|rc)\\d+)?$" : "$1.$2",
     "^v8.5.0$" : "main"
   }
-}
+}