Fix flaky iterations_finder_spec

What does this MR do?

ee/spec/finders/iterations_finder_spec.rb was flaky due to not setting the due_date for an Iteration in one of the specs.

The reason this caused a problem is because when not given a date, the factory falls back on using an incremental counter for the date, which has a high chance of collision if you've been writing out dates by hand.

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
• 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

Closes #218029 (closed)