Add newline between the command and the output when collapsible section is enabled

What does this MR do?

Add newline after the section start and section end.

Before	After

Why was this MR needed?

In !3014 (merged), echo was replaced by printf. By default, the former add a trailing newline, the latter doesn't.

What's the best way to test this MR?

Create this job:

# .gitlab-ci.yml
test_script-section:
  script:
    - echo foo
  needs: []
  variables:
    FF_SCRIPT_SECTIONS: "true"

Result before: https://gitlab.com/tchandelle/demo-pipeline-job-dependencies/-/jobs/2266467248#L27

Result after: https://gitlab.com/tchandelle/demo-pipeline-job-dependencies/-/jobs/2266483824#L26

What are the relevant issue numbers?

Closes #28969 (closed)