Bump `jupyter-scheduler` to `v2.7.0` #291

Job	Run time
build-test-images (jupyterlab, gpu)	3m 48s
build-test-images (jupyterlab, cpu)	3m 43s
build-test-images (jupyterhub, cpu)	2m 28s
build-test-images (dask-worker, gpu)	1m 37s
build-test-images (dask-worker, cpu)	1m 35s
	13m 11s

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