Adds `.json` to the end of pipelines endpoint

What does this MR do?

The problem is explained here:

Why are we not using the .json in the url? But sending the application/json header instead for the json response? This is most likely the reason it gets cached because we use https://gitlab.com/gitlab-org/gitlab-ce/pipelines/32543288 not https://gitlab.com/gitlab-org/gitlab-ce/pipelines/32543288.json so when the user clicks the back button the browser sees it's the same url so sends over the Etag and caching takes effect. I think we just append .json for the request of the pipelines it should solve this problem.

This MR adds .json to the endpoint. We had this issue in the past, when we first started moving things into Vue and I forgot about it.

What are the relevant issue numbers?

Closes #52472 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides

changelogs/unreleased/52472-pipeline-endpoint-json.yml

 ---
 title: Fix caching issue with pipelines URL
-merge_request:
+merge_request: 22293
 author:
 type: fixed