xls/github: add dependabot configuration

- enable newly released bazel support
- setup weekly update

See https://github.blog/changelog/2025-12-16-dependabot-version-updates-now-support-bazel/

PiperOrigin-RevId: 849154640

Author: proppy

.github/dependabot.yml

@@ -0,0 +1,6 @@
+version: 2
+updates:
+  - package-ecosystem: "bazel"
+    directory: "/"
+    schedule:
+      interval: "weekly"