Force dependabot to ignore grpc

Upgrading gRPC automatically has caused regressions, therefore this
should be done manually. If the offending breakage is resolved, we can
remove this and use extra vigilance enforced by CI to detect breakage.

.github/dependabot.yaml

@@ -31,6 +31,7 @@ updates:
     labels: ["lang/go", "area/dependencies"]
     ignore:
       - dependency-name: "github.com/inspektor-gadget/inspektor-gadget"
+      - dependency-name: "google.golang.org/grpc"
     open-pull-requests-limit: 10
   - package-ecosystem: "gomod"
     directory: "/hack/tools"