Revert deprecation of certain fields in decision event

## What are the schemas that are affected by the issue
- https://github.com/adobe/xdm/blob/master/extensions/adobe/experience/decisioning/proposition-detail.schema.json
- https://github.com/adobe/xdm/blob/master/extensions/adobe/experience/decisioning/decisionevent.schema.json
## What are examples of products that are impacted by the issue
- Offer Decisioning

Changes:
- [This](https://github.com/adobe/xdm/blob/master/extensions/adobe/experience/decisioning/decisionevent.schema.json#L24) field needs to be marked as experimental as it is future facing.
- From this [schema](https://github.com/adobe/xdm/blob/master/extensions/adobe/experience/decisioning/proposition-detail.schema.json), the following fields are currently marked as deprecated, which needs to be reverted: `xdm:activity`, `xdm:placement`, `xdm:selections`, `xdm:fallback`.

Inspect decisionevent.schema.json at the linked field around line 24 and proposition-detail.schema.json, focusing on the listed deprecation and experimental annotations. Done means the decision event field is marked experimental and xdm:activity, xdm:placement, xdm:selections, and xdm:fallback are no longer marked deprecated.

Written by the indexing model from the issue text.