Add a default issue template and update default PR template

[aliok]

Related knative/community#1251

Add a default issue template and update default PR template with the adopter form information.

[knative-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[aliok]

/close

Wrong repo

[knative-prow]

@aliok: Closed this PR.

In response to this:

/close

Wrong repo

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[aliok]

Created knative-extensions/.github#252 instead