gossipd: Correct evaluation of new channel announcement timestsamp

[endothermicdev]

According to the description, we should be checking the existence of a channel update on the other side here.

[!IMPORTANT] Open for merging new PRs until the next PR freeze date is confirmed!

Checklist

Before submitting the PR, ensure the following tasks are completed. If an item is not applicable to your PR, please mark it as checked:

• [ ] The changelog has been updated in the relevant commit(s) according to the guidelines.
• [ ] Tests have been added or modified to reflect the changes.
• [ ] Documentation has been reviewed and updated as needed.
• [ ] Related issues have been listed and linked, including any that this PR closes.