update backport file (#665)

(cherry picked from commit 7a06e45)

Author: mongoKart

.backportrc.json

@@ -2,14 +2,35 @@
   // Required
   "repoOwner": "mongodb",
   "repoName": "docs-node",
-
   // the branches available to backport to
-  "targetBranchChoices": ["master", "v5.2", "v5.1", "v5.0", "v4.15", "v4.14", "v4.13", "v4.12", "v4.11", "v4.10", "v4.9", "v4.8", "v4.7", "v4.6", "v4.5", "v4.4", "v4.3", "v4.2", "v4.1", "v4.0", "v3.7", "v3.6"],
-
+  "targetBranchChoices": [
+    "master",
+    "v5.3",
+    "v5.2",
+    "v5.1",
+    "v5.0",
+    "v4.15",
+    "v4.14",
+    "v4.13",
+    "v4.12",
+    "v4.11",
+    "v4.10",
+    "v4.9",
+    "v4.8",
+    "v4.7",
+    "v4.6",
+    "v4.5",
+    "v4.4",
+    "v4.3",
+    "v4.2",
+    "v4.1",
+    "v4.0",
+    "v3.7",
+    "v3.6"
+  ],
   // Optional: automatically merge backport PR
   "autoMerge": true,
   "autoMergeMethod": "squash",
-
   // Optional: Automatically detect which branches a pull request should be backported to based on the pull request labels.
   // In this case, adding the label "auto-backport-to-production" will backport the PR to the "production" branch
   "branchLabelMapping": {