Add configuration for sorenlouv/backport tool (#4474)

(cherry picked from commit 0c9e661)

Author: pquentin

.backportrc.json

@@ -0,0 +1,6 @@
+{
+  "repoOwner": "elastic",
+  "repoName": "elasticsearch-specification",
+  "targetBranchChoices": ["9.0", "8.19", "8.18", "8.17"],
+  "fork": false
+}