Idempotency of change variables batch operation

[joaquinfelici]

User Story (Required on creation)

As a user, executing a batch operation for setting variables in cockpit should always succeed even if one or more process instances have been completed/deleted in the meantime.

Functional Requirements (Required before implementation)

• When executing a batch operation for setting variables on one or more process instances, if any of the process instances has been completed/deleted, the batch should still succeed.
• Same behavior when calling the set variables async rest api endpoint.

Technical Requirements (Required before implementation)

• Introduce mechanism in AbstractVariableCmd to not fail on given conditions (execution coming from batch).
• Default behavior (for sync api) will be to fail as usual.
• Handle new behavior in BatchSetVariablesHandler only.

Limitations of Scope

• Only for async api.
• No configurable flag is needed.

Hints

• Unit testing.
• Documentation for java/rest api.
• Migration guide should mention this change in behavior for batch.

Links

• https://github.com/camunda/product-hub/issues/2512

Breakdown

### Pull Requests
- [ ] https://github.com/camunda/camunda-bpm-platform/pull/4790
- [ ] https://github.com/camunda/camunda-docs-manual/pull/1707
- [ ] https://github.com/camunda/camunda-docs-manual/pull/1708
- [ ] https://github.com/camunda/camunda-bpm-platform-maintenance/pull/1314

Dev2QA handover

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