Early branch: Adjust the verify_ce job on the final release candidate

When testing the final release candidate creation #21327 (closed). The verify_ce job failed because it couldn't find the Add latest changes from gitlab-org/gitlab@18-2-stable-ee commit: https://ops.gitlab.net/gitlab-org/release/tools/-/jobs/19306150

2025-07-15 20:48:41.633613 W ReleaseTools::Monthly::ReleaseCandidate::VerifyStableBranchCommits -- Next commit after version update doesn't have expected message -- {commit_id: "5350320d526da1f490c8c995dda49179dbb2581d", message: "Ensure pipelines run for 18.2.0-rc43\n\nThe last commit synced to this repository includes the \"ci skip\" tag,\npreventing tag pipelines from running. To ensure these pipelines do run,\nwe create this commit. This normally is only necessary if a Merge Train\nsync didn't introduce any new changes.\n\nFor more information, refer to issue\nhttps://gitlab.com/gitlab-com/gl-infra/delivery/-/issues/1355.", expected_messages: ["Add latest changes from gitlab-org/gitlab@18-2-stable-ee", "Ensure pipelines run for v18.2.0-rc"]}
2025-07-15 20:48:41.633665 E ReleaseTools::Monthly::ReleaseCandidate::VerifyStableBranchCommits -- CE stable branch verification failed -- {error: "Could not find valid commit after version update on 18-2-stable"}
2025-07-15 20:48:41.633778 I ReleaseTools::Slack::ReleaseJobEndNotifier -- Posting slack message -- {job_type: "Verify CE stable branch commits", status: :failed}
2025-07-15 20:48:41.885850 F Rake::Task -- Task failed -- Exception: RuntimeError: Could not find valid commit after version update on 18-2-stable

When tagging subsequent release candidates, it is likely that message won't be present since there is not enough time to sync changes from GitLab to GitLab FOSS. For example, have a look at the commit timeline on the 18.1 stable branches, where multiple release candidates (42, 43, 44) were tagged after each other:

https://gitlab.com/gitlab-org/gitlab-foss/-/commits/18-1-stable

The Add latest changes from gitlab-org/gitlab@18-2-stable-ee should only be validated on the initial release candidate