Fix .gitignore file

- Adds missing 7.9 branch
- Makes 7.17 the default/checked branch

.backportrc.json

@@ -1,5 +1,5 @@
 {
   "upstream": "elastic/security-docs",
-  "branches": [{ "name": "7.16", "checked": true }, "7.15", "7.14", "7.13", "7.12", "7.11", "7.10", "7.8"],
+  "branches": [{ "name": "7.17", "checked": true }, "7.16", "7.15", "7.14", "7.13", "7.12", "7.11", "7.10", "7.9", "7.8"],
   "labels": ["backport"]
 }