Demote `object not found` error to warning in Gitleaks

Description

Secret detection job failing on www-gitlab-com with object not found during some MRs. This error is encountered when gitleaks cannot find a commit from a list generated by this git log command in the SD vendoered template:

git log --left-right --cherry-pick --pretty=format:"%H" refs/remotes/origin

Since the git log command is a best guess at determining commits, there are cases that pop up when the commit object is not found. We should demote the object not found error to a warning to avoid failed runs.

Tasks

• change the object not found error to a warning log in Gitleaks
• update the Secrets analyzer