generate migration guide for Az 12.0.0 (Azure#24877)

* generate migration guide for Az 12.0.0

* Update documentation/migration-guides/Az.12.0.0.0-migration-guide.md

Co-authored-by: Yeming Liu <[email protected]>

* update Az.Compute part format

* Update documentation/migration-guides/Az.12.0.0.0-migration-guide.md

Co-authored-by: yifanz7 <[email protected]>

* update according to comments

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Co-authored-by: Yeming Liu <[email protected]>
Co-authored-by: yifanz7 <[email protected]>

Author: 3 people