[typespec-ts] Adopt the latest customization process

As offline discussed we have following agreements regarding the customization tool:
- Remove v1 of the customization tool and remove the "v2" from the v2 tool
- Move all existing uses of customization to the v2 tool and adopt a consistent structure
- Update the documents: https://github.com/Azure/azure-sdk-for-js/wiki/Migrating-to-TypeSpec#step-7-apply-customizations

For codegen side, we need to ensure to run customization commands in automation side.

Relevant issues:
- https://github.com/Azure/autorest.typescript/issues/3454
- https://github.com/Azure/autorest.typescript/issues/3325