Set k8s default container label

[konstfish]

Fixes #15391

Proposed Changes

• Add the kubectl.kubernetes.io/default-container annotation to Pods, pointing to the user-container. This ensures kubectl will always select the correct pod by default when interacting with kubectl logs, kubectl exec, etc.

Release Note

Add default container annotation to Pods created by the reconciler

[linux-foundation-easycla]

• ✅login: konstfish / (f154801)
• ✅login: konstfish / (f154801, cf0d7fc)
• ✅login: konstfish / (f154801, cf0d7fc, f154ef9)
• ✅login: konstfish / (f154801, cf0d7fc, f154ef9, f1540e2)
• ✅login: konstfish / (f154801, cf0d7fc, f154ef9, f1540e2, f1545d9)

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[knative-prow]

Welcome @konstfish! It looks like this is your first PR to knative/serving 🎉

[knative-prow]

Hi @konstfish. Thanks for your PR.

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[knative-prow]

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[konstfish]

Some more notes on this, I was unsure if I should put this behind some sort of feature gate, but I'd be happy to as about ~12 tests were affected by these changes. Fix for now was just to include the extra annotation in the want (in https://github.com/knative/serving/pull/15694/files#diff-d93eacd83502ff2061608160378ff8c5c8b199fed97cd44e2b1ab73f419b9aab & https://github.com/knative/serving/pull/15694/files#diff-508e07fc6207ae8956d33f7753a1407b85885edff3135849635263501ed77fd5) but I feel like this isn't really the cleanest solution. Would love some feedback on that!