Porting old Umbraco API Controller [Relation Service]

[erikjanwestendorp]

Description

This PR belongs to: https://github.com/umbraco/UmbracoDocs/issues/6466.

• [x] Change UmbracoApiController to Controller
• [x] Remove using statement using Umbraco.Cms.Web.Common.Controllers;
• [x] Add Attributes
• [x] Change IComponent to NotificationHandler

Type of suggestion

• [ ] Typo/grammar fix
• [x] Updated outdated content
• [ ] New content
• [ ] Updates related to a new version

Product & version (if relevant)

Umbraco CMS v14 & 15

[sofietoft]

Thanks @erikjanwestendorp ! 💪

I'll make sure we review this as soon as possible.

[erikjanwestendorp]

@sofietoft Just updated the PR with your suggestion 😄