Update logstash docs to object notation for data_stream

[simitt]

Since elastic/apm-data#124 the apm-server sends the data_stream information in object notation rather than dotted fields. This aligns better with logstash expectations.

Update the logstash guide to remove the mutate filter as it is not necessary anymore.

[lucabelluccini]

Thanks Silvia.
I would "keep it" but just prepend it with a "if you're using a previous version of APM Server, please add this mutate filters..." or the like to guarantee some backward compatibility for mixed versions.

[simitt]

The docs for previous versions would still contain the section, but the docs since 8.10, where the behavior changed wouldn't. IMO it'd be more confusing to discuss workarounds in a version where they are not relevant anymore.

[bmorelli25]

@colleenmcginnis, sorry for not seeing this earlier. Would've been super easy to add this onto #11989.

There's another Logstash update requested in #11651. Could probably update both in a single PR.