Explicitly define base branches

Currently, the dependency dashboard lists a bunch of pending updates
under a section called "Other branches". I'm not sure, but this sounds
like one of the configs I extend from enables base branches other than
only the default. To test, and make it explicit, set only the branches
I really want checked.

I'm adding the `release/.*` regex for now, even though I don't have
any release process yet.

.renovaterc

@@ -7,5 +7,9 @@
     ":rebaseStalePrs"
   ],
   "prConcurrentLimit": 10,
-  "branchPrefix": "renovate/"
+  "branchPrefix": "renovate/",
+  "baseBranches": [
+    "$default",
+    "/^release\\/.*/"
+  ]
 }