Update Changelog workflow fails when running for PR from repo fork

[DannyvdSluijs]

Update Changelog workflow fails when running for PR from repo fork. See the workflow run for #904: https://github.com/jsonrainbow/json-schema/actions/runs/25362058281

Some debugging hinted to update the event to pull_request_target but this comes with some risks which should be considered before "just" changing the event. See https://docs.github.com/en/actions/reference/workflows-and-actions/events-that-trigger-workflows#pull_request_target

Risk mitigation options to check:

• Use sparse checkout any only fetch CHANGELOG.md
• None or minimal execution of scripts
• No evaluation of environment variables to avoid shell injection, see Fix shell injection in update-changelog workflow for PR titles with backticks #897