Context:
To avoid having to handle decision and process report definitions differently in the recent changes on the process definition with OPT-4825 we agreed to align the decision report definition to also switch the `view.property` field to `view.properties`.

AT:

• a new field view.properties of type array is added to align this report type with the recent process data changes
• the current singular field view.property is kept for backwards compatibility, will be removed with OPT-4872
• existing reports are migrated using the upgrade script
• REST API documentation in Confluence is updated