fix default pvc mounting with kubespawner 7 #2604

Job	Run time
lint_and_validate_rendered_templates	9s
lint_and_validate_templates_with_schema	5s
lint_shell_scripts	56s
lint_and_validate_templates_with_schema (v3.5.0)	26s
test (latest, install)	1m 43s
test (stable, install, --set hub.image.name=quay.io/jupyterhub/k8s-hub-slim --set prePuller.hook....	2m 8s
test (v1.31, install, --set hub.existingSecret=test-hub-existing-secret --set proxy.secretToken=a...	1m 43s
test (v1.30, upgrade, stable, --set proxy.secretToken=aaaa1111 --set hub.cookieSecret=bbbb2222 --...	2m 46s
test (v1.29, upgrade, dev, --set proxy.secretToken=aaaa1111 --set hub.db.type=sqlite-pvc --set si...	2m 37s
test (v1.28, upgrade, 2.0.0, --set proxy.secretToken=aaaa1111 --set hub.cookieSecret=bbbb2222 --s...	3m 12s
	15m 45s

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