[Dt]: [Epic]: Convert open enumerations to unions

[markcowl]

Tasks

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1. [Laurent]: Convert data-plane specs in azure-rest-api-specs/main
2. [Tim] Add union for open enum rule to Azure.Core all ruleset Azure Core: Enable no-enum rule by default in all ruleset #467
3. Enable no-closed-literal rule #505
   +0
4. Enums are always fixed after we switch to use union to represent extensible enum #459
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5. Migrate enum to union in samples #500
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6. Add troubleshoot guide for enum -> union migration #506
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7. [TypeSpec]: Convert enums to unions for library types (Azure.Core, Azure.ResourceManager)
8. Enums are not extensible by default anymore #473
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9. Add Azure.Core linter rule to encourage open union for discriminator #97
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10. Remove non relevant linter rules #707
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11. Update docs to reference union instead of unions #706
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12. [TypeSpec] Deprecate @fixed