Update deployment ref validation

What does this MR do?

A successful deployment triggers AfterCreateService#execute. In some cases, this can fail if the related source reference does not exist any longer. As a result, when AfterCreateService#execute tries to create a new ref based on the source ref, it'll fail.

To fix this case, a validation was added here in this MR.

Screenshots

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

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

Closes #193170 (closed)