migrate: bump extension version to 3.0.0b6

[krdhruva]

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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 azdev 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.

[Copilot]

Copilot review overview

🟢 Approval recommended

The focused payload correction is consistent with its callers and adequately covered by unit tests.

Review effort: Balanced
Findings: None

What changed in this PR

Bumps the Migrate extension to 3.0.0b6 and fixes execution-level runbook action payloads.

Changes:

• Updates the extension version and release notes.
• Omits invalid empty targetId values from execution actions.
• Updates unit tests for the corrected payload.

File	Description
src/​migrate/​setup.py	Bumps the extension version.
src/​migrate/​HISTORY.rst	Summarizes new runbook commands.
src/​migrate/​azext_migrate/​runbook/​models.py	Corrects action payload construction.
src/​migrate/​azext_migrate/​tests/​latest/​runbook/​test_runbook_unit.py	Verifies corrected payloads.

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[Azure CLI Bot (azclibot)]

[Release] Update index.json for extension [ migrate-3.0.0b6 ] : https://dev.azure.com/msazure/One/_build/results?buildId=183281134&view=results