WIP: passing source and target ref to downtime_check rake task

What does this MR do?

Updates the downtime_check rake task to accept source and target refs for getting the changed files list for the specific MR being run.

We're getting pipeline failures on merge requests to -stable branches because we diff the MRs from a blank ref, which defaults to master. These changes passes source and target SHA in our CI config, which will now be accepted and used by the downtime_check rake task.

I haven't written any of our own CI config, so I'm not sure what the processes are for validating this change other than "it doesn't break"

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Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

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