[Umbrella] Publish release-1.18

<!-- Please only use this template for submitting enhancement requests --> **What would you like to be added**: According to [Karmada's release cycle](https://karmada.io/docs/releases#the-release-cycle), the release team is preparing to release v1.18.0 at the end of the month(2026-05-30), along with patch versions for the maintenance releases. This issue is to submit the release publishing tasks. karmada repo: - [ ] prepare release notes - [ ] release the v1.18.0, v1.17.3, v1.16.6, v1.15.9 (@RainbowMango ) - [ ] Stop maintaining version 1.15 and maintain version 1.18 - update ci-schedule-compatibility.yaml URL: https://github.com/karmada-io/karmada/blob/master/.github/workflows/ci-schedule-compatibility.yaml - update dependabot.yaml URL: https://github.com/karmada-io/karmada/blob/master/.github/dependabot.yml - update ci-image-scanning-on-schedule.yml URL: https://github.com/karmada-io/karmada/blob/master/.github/workflows/ci-image-scanning-on-schedule.yml - [ ] Add helm index v1.18.0 (auto by CI) - [ ] Add helm index v1.17.3 (manually trigger the CI, @RainbowMango ) - [ ] Update [Kubernetes compatibility](https://github.com/karmada-io/karmada/blob/master/README.md#kubernetes-compatibility) website repo: - [ ] Add upgrading v1.17 to v1.18 docs - [ ] Update Reference Documents - [ ] Release docs for release-1.18 - [ ] Stop maintaining v1.12 - [ ] release branch release-1.18 (@RainbowMango ) **Why is this needed**: