chore: enable dependabot for git submodules (#775)

This will automatically create PRs when any of the git submodules has a
new version available.

Author: tautschnig

.github/dependabot.yml

@@ -0,0 +1,11 @@
+# Copyright Kani Contributors
+# SPDX-License-Identifier: Apache-2.0 OR MIT
+
+# https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file
+
+version: 2
+updates:
+  - package-ecosystem: "gitsubmodule"
+    directory: "/"
+    schedule:
+      interval: "weekly"