Deprecate `defaultProject`

[dgp1130]

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Description

In #11111 (comment), we decided to deprecate defaultProject but never got around to doing it. The use case it provided for is better solved by creating an empty monorepo and then generating an application. We did have a PR to do this, but it is pretty out of date now and not very usable.

Describe the solution you'd like

Deprecate (and eventually remove) defaultProject.

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