Include squash and diff head SHA in Chain of Custody report

What does this MR do?

The Chain of Custody report in the Compliance Dashboard reports only Merge Commit SHA for the Merge Commit, while the Compliance Dashboard lists fast-forwarded MRs.

This MR rectifies it by reporting the merged_commit_sha - which could include merge_commit_sha or squash_commit_sha or diff_head_sha in that order of preference.

Mentions #267628 (closed)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team