Update to serving 0.6.0

[rhuss]

This PR makes an update to knative-serving 0.6.0 by

• Updating go.mod dependency to 0.6.0
• Updating code and tests to still use github.com/knative/serving/pkg/apis/serving/v1alpha1 but with Depectation references update + fix for change from value to pointer types in the serving types.

This change should work now for both 0.5.x and 0.6.0 installations.

Next step would be then to switch the new API but still supporting the v1alpha1 version if this is the highest version supported by the server.

[knative-prow-robot]

Hi @rhuss. Thanks for your PR.

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[cppforlife]

/ok-to-test

[sixolet]

/approve
/lgtm

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: rhuss, sixolet

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