Add the merge date to the Chain of Custody report

What does this MR do and why?

This MR adds an additional column to the Chain of Custody report as described in issue 365041

This new column is the merged_at date from the associated merge request

This commit also:

• updates the related spec
• updates the related docs

Screenshots

How to set up and validate locally

1. Have gitlab running locally
2. On the top bar, select Menu > Groups and find the gitlab group.
3. On the left sidebar, select Security & Compliance > Compliance report.
4. Select List of all merge commits.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.