Allow admins to merge topics [API]

What does this MR do and why?

This MR is the third step of #366884 (closed):

• Extend the Topics API to allow admins to merge topics

The Topics::MergeService was already implemented in a previous MR and the UI part is done in another MR.

🛠 with ❤ at Siemens

/cc @bufferoverflow

How to set up and validate locally

1. See API documentation: https://docs.gitlab.com/ee/api/topics.html
2. Create some topics
3. Merge topics:

curl --request POST \
     --data "source_topic_id=<ID of source topic>&target_topic_id=<ID of target topic>" \
     --header "PRIVATE-TOKEN: <your_access_token>" \
     "https://gitlab.example.com/api/v4/topics/merge"

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.