Make Dependabot to ignore connectedhomeip

It seems Dependabot can't handle submodules with recursive submodules
well: It fetches them recursively and ultimately runs out of space.

See also: dependabot/dependabot-core#9376

Author: agners

.github/dependabot.yml

@@ -2,7 +2,7 @@ version: 2
 updates:
   - package-ecosystem: "gitsubmodule"
     directory: "/"
-    allow:
+    ignore:
       - dependency-name: "connectedhomeip"
     schedule:
       interval: "daily"