Rename `messaging.kafka.message.offset` to `messaging.kafka.offset`

[pyohannes]

Fixes #1156

Changes

As discussed in #1156, the offset property in Kafka can be used independent of a single message, but also describe the position in the overall message stream, and thus denote whole sections of a message stream. For example by advancing a checkpoint or offset, one can denote a batch of consecutive messages as checkpointed.

To avoid unnecessary confusion around this, the workgroup agreed to move the offset attribute out of the message-specific sub-namespace.

Merge requirement checklist

• [x] CONTRIBUTING.md guidelines followed.
• [x] Change log entry added, according to the guidelines in When to add a changelog entry.
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• [x] schema-next.yaml updated with changes to existing conventions.