Logs should have `spanId` as a property on `SentryLogEvent`

[adinauer]

Description

For metrics, we're putting spanId as a top level property on SentryMetricsEvent.
For logs it's currently an attribute - we should change that.
We can simply move from attribute to top level property without a major - no need to write to both since relay handles both cases.

[linear]

JAVA-271