Revert "Remove 8.17 from dev branches" (elastic#131880)

This reverts commit f0bb49e.

(cherry picked from commit 48cb0ef)

Author: brianseeders

branches.json

@@ -10,6 +10,9 @@
     {
       "branch": "8.18"
     },
+    {
+      "branch": "8.17"
+    },
     {
       "branch": "7.17"
     }