Ensure timestamps are set for manually created vulnerabilities

What does this MR do and why?

With #433505 (closed), it was discovered that some vulnerabilities exist which are in the dismissed state and do not have a dismissed_at timestamp, which is unexpected. Vulnerabilities::ManuallyCreateService does not validate that the relevant timestamp is present when a vulnerability is created in that respective state. This MR fixes the issue by setting the timestamp to the current time if it wasn't provided in the params.

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