(GKE): Update documentation about calling inference service endpoing behind IAP. Fix https://github.com/kubeflow/gcp-blueprints/issues/324

[zijianjoy]

What this PR does / why we need it:

Validated that GCP IAP instruction here is working with Kubeflow 1.4 release.

Related GitHub issues:

fixes #1660
#913
#1660
#1199

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes GoogleCloudPlatform/kubeflow-distribution#324

Special notes for your reviewer:

1. Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

No change

Release note:

NONE

[aws-kf-ci-bot]

Hi @zijianjoy. Thanks for your PR.

I'm waiting for a kserve member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[yuzisun]

/lgtm
/approve

[kserve-oss-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: yuzisun, zijianjoy

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[yuzisun]

/lgtm

thanks @zijianjoy !

[yuzisun]

/ok-to-test