fix(@angular/cli): fix schema error Property targets is not allowed

[alan-agius4]

Closes #12192

[googlebot]

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[clydin]

either one (but not both at the same time) should be allowed for 7.x. Unfortunately, this makes the schema far more complicated.

[alan-agius4]

This is because targets and architect are not really required. If they were it could have been something like

"oneOf": [
        {
            "required": ["targets"]
         },
        {
            "required": ["architect"]
        }
]

[googlebot]

☹️ Sorry, but only Googlers may change the label cla: yes.

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