Update unique field creation mechanism

[FelixMalfait]

Currently the indexes for unique fields are set at the decorator level:

https://github.com/twentyhq/twenty/blob/main/packages/twenty-server/src/engine/twenty-orm/decorators/workspace-is-unique.decorator.ts#L20

Instead, we would like to remove that code at the index level and take care of it in the workspace migration runner service, kind of like what we did here: https://github.com/twentyhq/twenty/blob/main/packages/twenty-server/src/engine/workspace-manager/workspace-migration-runner/workspace-migration-runner.service.ts#L516