hyperkube removal tracking (after v1.18 EOL)

[justaugustus]

The next v1.18 release (v1.18.19) on 5/7/21 will bring v1.18 to End of Life: kubernetes/sig-release#1532
With that, we'll need to remove any remaining references to hyperkube.

This issue tracks that work.

• Remove debian-hyperkube-base image building:
  • Remove hyperkube references #2018
  • Remove deprecated hyperkube image building jobs test-infra#21941
• area/hyperkube label: Remove deprecated hyperkube image building jobs test-infra#21941
• gcb/build prototype jobs: gcb: Remove build prototype job #2019
• Remove hyperkube from subproject listing: sig-release: Remove the deprecated hyperkube subproject from listing community#5850

[BenTheElder]

cc @dims

[justaugustus]

Last piece: kubernetes/community#5850
/assign
/area release-eng hyperkube
/sig release
/kind cleanup

[k8s-ci-robot]

@justaugustus: The label(s) area/hyperkube cannot be applied, because the repository doesn't have them.

In response to this:

Last piece: kubernetes/community#5850
/assign
/area release-eng hyperkube
/sig release
/kind cleanup

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