Add only_new_paths option to Tooling::FindChanges

What does this MR do and why?

When a file in the documentation is moved, the old path was also passed to markdownlint and vale. That causes the linting to fail, because the files don't exist. For example: https://gitlab.com/gitlab-org/gitlab/-/jobs/5888800308

In order to fix that, the new option will only return the new paths of the changed files and not the old paths.

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Tested with the changes of !119310 (diffs)

export CI_API_V4_URL=https://gitlab.com/api/v4
export CI_MERGE_REQUEST_PROJECT_PATH=gitlab-org/gitlab
export CI_MERGE_REQUEST_IID=119310
scripts/lint-doc.sh

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