BiDiKo: Mark the event `forward-submission` as deprecated in the new SET-payload release (BiDiKo-Release) (1.2.0)

In preparation for #310, the event forward-submission should be marked as deprecated. This change should be implemented with the next SET-payload release 1.2.0 (contains BiDiKo changes).

Akzeptanzkriterien

1. Event forward-submission is marked deprecated in SET-payload.
2. CHANGELOG of SET-payload has an entry describing that the event is deprecated.

Durchführungsplan (von Entwickler:in bei Umsetzungsplanung auszufüllen)

• Mark the event forward-submission as deprecated in the new SET-payload. --event-payload!10 (merged)
• Release new release candidate (rc) of 1.2.0. → 1.2.0-rc1
• Update CHANGELOG. → event-payload!10 (merged)