Deprecate Versioning APIs previous_* response fields (#669)

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<!-- Describe what has changed in this PR -->
Deprecating the following fields in favor of API idempotency.
-
`SetWorkerDeploymentCurrentVersionResponse.previous_deployment_version`
-
`SetWorkerDeploymentRampingVersionResponse.previous_deployment_version`
- `SetWorkerDeploymentRampingVersionResponse.previous_percentage`
- `SetWorkerDeploymentManagerResponse.previous_manager_identity`

<!-- Tell your future self why have you made these changes -->
The fields do not fit well with these idempotent APIs. They will return
invalid results when the API is retried by server or client.

Users who need this info can use `DescribeWorkerDeployment` before
calling this the `Set*` APIs. By passing the `conflict_token` got from
the `DescribeWorkerDeployment` call to the Set API they can ensure there
is no interfering changes between the two calls.

<!-- Are there any breaking changes on binary or code level? -->
**Breaking changes**
These fields are deprecated and will stop being populated sometime after
Versioning GA.

<!-- If this breaks the Server, please provide the Server PR to merge
right after this PR was merged. -->
**Server PR**
No server change.

Author: ShahabT