Use project iterations query for project iterations list

What does this MR do?

Iterations at a group level has now been released. Issue #222763 (closed) aims to show iterations at project level.

This MR uses the project iterations query instead of the group iterations query for the project iterations list.

These changes are behind the feature flag project_iterations, defaulted to off.

This MR is part of a sequence of changes:

1. An MR to add a bare Iterations page in projects (!37715 (merged))
2. An MR to show a list of iterations on the Iterations page in projects (!37961 (merged))
3. An MR to show an iteration report within a project (!37979 (merged))
4. An MR to use the project iterations query instead of group iterations query for project iterations list (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
• 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