TASK: Use collection value objects in content graph instead of `iterable`

[mhsdesign]

Solves partially: https://github.com/neos/neos-development-collection/issues/5102

In the content graph we have some methods returning NodeAggregates and others iterable<NodeAggregate> while it seems like it, the iterable is also not lazy and yielded but fetches all nodes eagerly so there is no performance / memory gain.

This change is not really breaking, as NodeAggregates implement \IteratorAggregate and thus foreaching or calling iterator_to_array on the results will not make a difference.

Though this change should happen before a final release :D

Upgrade instructions

Review instructions

Checklist

• [ ] Code follows the PSR-2 coding style
• [ ] Tests have been created, run and adjusted as needed
• [ ] The PR is created against the lowest maintained branch
• [ ] Reviewer - PR Title is brief but complete and starts with FEATURE|TASK|BUGFIX
• [ ] Reviewer - The first section explains the change briefly for change-logs
• [ ] Reviewer - Breaking Changes are marked with !!! and have upgrade-instructions