Bump aks-preview version to 19.0.0b21 for rollback feature release

[InduSridhar]

---

This checklist is used to make sure that common guidelines for a pull request are followed.

Related command

General Guidelines

• Have you run azdev style <YOUR_EXT> locally? (pip install azdev required)
• Have you run python scripts/ci/test_index.py -q locally? (pip install wheel==0.30.0 required)
• My extension version conforms to the Extension version schema

For new extensions:

• My extension description/summary conforms to the Extension Summary Guidelines.

About Extension Publish

There is a pipeline to automatically build, upload and publish extension wheels.
Once your pull request is merged into main branch, a new pull request will be created to update src/index.json automatically.
You only need to update the version information in file setup.py and historical information in file HISTORY.rst in your PR but do not modify src/index.json.

[azure-client-tools-bot-prd]

️✔️Azure CLI Extensions Breaking Change Test

️✔️Non Breaking Changes

[yonzhan]

Thank you for your contribution! We will review the pull request and get back to you soon.

[github-actions]

The git hooks are available for azure-cli and azure-cli-extensions repos. They could help you run required checks before creating the PR.

Please sync the latest code with latest dev branch (for azure-cli) or main branch (for azure-cli-extensions).
After that please run the following commands to enable git hooks:

pip install azdev --upgrade
azdev setup -c <your azure-cli repo path> -r <your azure-cli-extensions repo path>

[github-actions]

CodeGen Tools Feedback Collection

Thank you for using our CodeGen tool. We value your feedback, and we would like to know how we can improve our product. Please take a few minutes to fill our codegen survey

[github-actions]

• For more info about extension versioning, please refer to Extension version schema

[Copilot]

Pull request overview

This PR bumps the aks-preview extension version from 19.0.0b20 to 19.0.0b21 to release the new rollback feature for AKS node pools. The version follows the beta versioning pattern used throughout the aks-preview extension.

Changes:

• Version bumped from 19.0.0b20 to 19.0.0b21 in setup.py
• Added 19.0.0b21 release section in HISTORY.rst documenting the new rollback commands

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/aks-preview/setup.py	Updates the VERSION constant to 19.0.0b21
src/aks-preview/HISTORY.rst	Adds a new version entry for 19.0.0b21 with changelog for the two new rollback commands

[yonzhan]

Please fix CI issues

[yanzhudd]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[azclibot]

[Release] Update index.json for extension [ aks-preview-19.0.0b21 ] : https://dev.azure.com/msazure/One/_build/results?buildId=151192259&view=results