fix: no events with case as subject

See merge request !12

the sub claim refers to the subject of the SET. previously
we had events whose subject is the case (case-close-event).

since we no longer have this event, the case subject is no longer
needed.

feat: Mark `forward-submission` as deprecated (planning#1292)

See merge request !10

New Rückkanal Events (planning#460)

See merge request !1

we decided to remove it since the status is also removed. so it
wouldn't make any more sense to keep it

since we expand the API to handle replies, we need the reply in the sub
claim. additionally, if we want to move away from using the 'txn' claim
and use the 'sub' claim for cases, it must be possible to specify them
in the sub claim. thats why case:<id> was added.

we either use Zustelldienst or (here), since its a technical specifier
the word "system"

Changed defintions to $def

See merge request !8

Replace $schema enum by $schema pattern

See merge request !5

The $schema attribute defines which schema versions are compatible with this schema. Software systems that validate a SET payload against this schema should not only accept SET payloads which exactly match this schema version but also accept SET payloads that validate against compatible schema versions (same major version)