[renovate] Match all branches instead of backporting for FTR group

FTR needs to be kept up to date across all branches, but using the
backport automation ends up requiring manual intervention due to merge
conflicts.

This configures renovate to open separate pull requests per branch.  If
this works as expected I'll do another pass and see if there's other
groups that this makes sense for.

renovate.json

@@ -17,6 +17,9 @@
   ],
   "baseBranches": [
     "main",
+    "8.x",
+    "8.17",
+    "8.16",
     "7.17"
   ],
   "prConcurrentLimit": 0,
@@ -2345,12 +2348,12 @@
         "team:kibana-operations"
       ],
       "matchBaseBranches": [
-        "main"
+        "*"
       ],
       "labels": [
         "Team:Operations",
         "release_note:skip",
-        "backport:all-open"
+        "backport:skip"
       ],
       "minimumReleaseAge": "7 days",
       "enabled": true