Adjust Vulnerabilties Slack integration to send only one message per pipeline

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Why are we doing this work

After working on Spike: Investigate failure to send expected Sla... (#367519 - closed) it turns out that we're being rate-limited by Slack (which is fair because we might be trying to send over 800 messages at once). Sending one message along the lines of pipeline X has discovered N vulnerabilities and link directly to the Security tab of the pipeline would be a better solution.

Implementation plan

TBA