Revert deprecation of certain fields in decision event

[salilkanetkar]

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 field needs to be marked as experimental as it is future facing.
• From this schema, the following fields are currently marked as deprecated, which needs to be reverted: xdm:activity, xdm:placement, xdm:selections, xdm:fallback.