chore: child pipelines no longer need to be cancelled explicitly

Description

Implements the new Gitlab pipeline cancellation feature. Cancelling the parent pipeline will automatically cascade to cancel all child pipelines. This means that the code that deals with fetching and cancelling child pipelines can be removed.

Supporting information

• gitlab-org/gitlab#273378 (closed)
• !35 (merged)
• https://tasks.opencraft.com/browse/SE-5459
• #13 (closed)

Testing instructions

1. Check these pipelines:

• Original pipeline: https://gitlab.com/opencraft/devstacks/keith/grove-aws/-/pipelines/741812396
• Abort pipeline: https://gitlab.com/opencraft/devstacks/keith/grove-aws/-/pipelines/741812782

2. For any running pipeline run the command ./grove abortpipeline [pipeline-id] and verify that the pipeline is cancelled
3. Repeat, but instead of running locally, use the gitlab trigger

Checklist

If any of the items below is not applicable, do not remove them, but put a check in it.

• All providers include the new feature/change
• All affected providers can provision new clusters
• Unit tests are added/updated
• Documentation is added/updated
• The TOOLS_CONTAINER_IMAGE_VERSION in ci_vars.yml is updated
• The grove-template repository is updated