Update Knative manifests to support newer kustomize versions

[berndverst]

Which issue is resolved by this Pull Request:
WIP for #1797

Description of your changes:

Kustomize 3.9.2 and beyond (including latest 4.X) does not seem to handle pointers / references in YAML anymore. Removing pointers appears to fix deployment problems with modern versions of Kustomize.

Can deploy with:

kustomize version
{Version:kustomize/v4.0.5 GitCommit:9e8e7a7fe99ec9fbf801463e8607928322fc5245 BuildDate:2021-03-08T20:53:03Z GoOs:darwin GoArch:amd64}

cd ~/src/berndverst/manifests/example
kustomize build --load-restrictor LoadRestrictionsNone .

Checklist:

• Unit tests pass:
  Make sure you have installed kustomize == 3.2.1
  1. make generate-changed-only
  2. make test

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This pull-request has been approved by: berndverst
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[davidspek]

duplicate of #1795

[yanniszark]

Thanks for the PR @berndverst!
Since this is a duplicate of #1795, let's continue the discussion there.