mgr/dashboard: fix "welcome" page reference in add-host e2e feature

mgr/dashboard: fix "welcome" page reference in add-host e2e feature

0fb99092ef2 renamed the cypress page-registry key "welcome" to
"onboarding" in urls.po.ts and updated 01-create-cluster-welcome.feature,
but missed the Background of 02-create-cluster-add-host.feature. so
urlsCollection.pages["welcome"] is undefined and the step fails with

  TypeError: Cannot read properties of undefined (reading 'url')
   at cypress/e2e/common/global.feature.po.ts:12

this fails the Background of every scenario in the spec, and cascades to
the later create-cluster specs that depend on the expanded cluster.

update the reference to "onboarding" to match the registry.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>