Properly mark `SyntheticsConfigVariable` `id` field as required

[api-clients-generation-pipeline[bot]]

See DataDog/datadog-api-spec#1192 Test branch datadog-api-spec/test/alaric.calmette/mark-variables-id-as-required

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[github-actions[bot]]

This issue has been automatically marked as stale because it has not had activity in the last 30 days. Note that the issue will not be automatically closed, but this notification will remind us to investigate why there's been inactivity.