Only prevent the unpublish or delete of a related item when configured to do so if it is related as a child, not as a parent (#18886)

* Only prevent the unpubkish or delete of a related item when configured to do so if it is related as a child, not as a parent.

* Fixed incorect parameter names.

* Fixed failing integration tests.

* Use using variable instead to reduce nesting

* Applied suggestions from code review.

* Used simple using statement throughout RelationService for consistency.

* Applied XML header comments consistently.

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Co-authored-by: mole <nikolajlauridsen@protonmail.ch>

src/Umbraco.Core/Models/RelationDirectionFilter.cs

@@ -0,0 +1,12 @@
+namespace Umbraco.Cms.Core.Models;
+
+/// <summary>
+/// Definition of relation directions used as a filter when requesting if a given item has relations.
+/// </summary>
+[Flags]
+public enum RelationDirectionFilter
+{
+    Parent = 1,
+    Child = 2,
+    Any = Parent | Child
+}