Upgrade istio to v1.21.3

[biswajit-9776]

Pull Request Template for Kubeflow manifests Issues

• Please include a summary of changes and the related issue.
• List any dependencies that are required for this change.
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✏️ A brief description of the changes

I upgraded istio from version 1.20.6 to 1.21.3

📦 List any dependencies that are required for this change

My PR depends on #

🐛 If this PR is related to an issue, please put the link of the issue here.

The following issues are related, because ...

✅ Unit Test Checklist

• 🛠️ Make sure you have installed kustomize == 5.2.1+
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[juliusvonkohout]

Please use this PR to finish your scripts. I want to use them myself for istio 1.22.

[biswajit-9776]

The scripts are ready to be used after this PR is merged.
For now I haven't simplified their order of execution; so to upgrade istio, you would have to do the following:

1. Have a kind cluster running which you can accomplish by running following command from manifests source directory.

kind create cluster --config tests/gh-actions/kind-cluster.yaml

2. You would need to run sync-istio-manifests.sh first. It might ask for your sudo password in between and then just push the branch for a PR.
3. After you have pushed the PR, you can now run the sync-istio-cni-manifests.sh.
4. Push the branch again and it's done.

[rimolive]

/lgtm

cc @juliusvonkohout

[biswajit-9776]

Sorry the script had a comment missing, which I have fixed now.

[juliusvonkohout]

The scripts are ready to be used after this PR is merged.
For now I haven't simplified their order of execution; so to upgrade istio, you would have to do the following:

1. Have a kind cluster running which you can accomplish by running following command from manifests source directory.

kind create cluster --config tests/gh-actions/kind-cluster.yaml

2. You would need to run sync-istio-manifests.sh first. It might ask for your sudo password in between and then just push the branch for a PR.
3. After you have pushed the PR, you can now run the sync-istio-cni-manifests.sh.
4. Push the branch again and it's done.

Please get rid of sudo and Kind. We just want to synchronize the manifests as we do in the other scripts e.g. sync-kserve.

[rimolive]

/lgtm

[juliusvonkohout]

/approve
since @rimolive reviewed it. Ill try to update to istio 1.22 with the scripts next week.
@rimolive you can upgrade to 1.22 as well if you have time.

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: juliusvonkohout

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[biswajit-9776]

@rimolive Do let me know if the scripts fail in any way.