Update DAST CONTRIBUTING.md with information about our review SLO

What does this MR do?

Within gitlab-org/secure/general#55 (closed) we are aiming to improve how we handle community contributions. One goal is to set expectations around our SLO (the time it will take to receive an initial response). This MR adds language to the CONTRIBUTING.md for DAST.

What are the relevant issue numbers?

gitlab-org/secure/general#55 (closed)

Does this MR meet the acceptance criteria?

Merge request reports

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