DOCSP-46924 Update C2C Backport file (#576)

Author: ajhuh-mdb

.backportrc.json

@@ -2,7 +2,7 @@
     "repoOwner": "mongodb",
     "repoName": "docs-cluster-to-cluster-sync",
 
-    "targetBranchChoices": ["master", "v1.9", "v1.8"],
+    "targetBranchChoices": ["master", "v1.10", "v1.9", "v1.8"],
 
     "editor": "code",