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

…lastic#205285)

FTR dependencies 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 for
the `ftr` group. 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