Change response for /ci/api/v1/builds/register.json from 404 to 204

## What does this MR do?

To check if there are new builds scheduled for a runner, runner is sending `POST /ci/api/v1/builds/register.json` requests. If there is a build then a `200` response with build data is returned. However if there is no builds scheduled for this runner it receives a `404 Not Found` response. This may end with a lot of `404 Not Found` lines in logs that are an expected behavior (please read #14445 for a reference).

Since `v1.0.1` version GitLab Runner is ready to receive a `204 No Content` response in case of no builds.

This MR adds a support for this status code for each Runner's version that can be determined while handling `POST /ci/api/v1/builds/register.json` request.

## Are there points in the code the reviewer needs to double check?

Specs in `spec/requests/ci/api/builds_spec.rb`

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] ~~API support added~~
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [ ] ~~Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)~~
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

Fixes #14445

See merge request !6225