Fixing some model references to configurable references

[dimitri-koenig]

There are some references to models which are not called via app('...') but directly. That means that overriding those references is not possible. This PR fixes that.

[what-the-diff[bot]]

PR Summary

• Replaced EloquentCollectionTree usage Replaced direct use of EloquentCollectionTree with a more flexible approach using app('statamic.eloquent.collections.tree') for better maintainability and customization.

• Simplified SubmissionModel date handling Refactored date handling in SubmissionModel with a more manageable and reusable approach by using app('statamic.eloquent.forms')::make()->fromDateTime($submission).

[jasonvarga]

Thank you