Revert "Remove 8.17 from branches.json" (elastic#132102)

This reverts commit afa7fec.

(cherry picked from commit 54d0e6f)

Author: brianseeders

branches.json

@@ -16,6 +16,9 @@
     {
       "branch": "8.18"
     },
+    {
+      "branch": "8.17"
+    },
     {
       "branch": "7.17"
     }