Ensure ecs.version is treated as a top level field

[Mpdreamz]

This ensures the ecs.version is always written as top level field without arbitrary nesting it.

This aligns the spec closer to the written documentation:

https://github.com/elastic/ecs-logging/tree/main/spec#minimum-viable-product

[apmmachine]

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• Start Time: 2023-01-18T15:35:09.538+0000

• Duration: 3 min 58 sec

[trentm]

Note (to self, I guess): I'll have to change the node.js ecs-logging libs to do this. Currently they are written log records like this:

{"log.level":"debug","@timestamp":"2023-01-19T17:51:47.649Z","log":{"logger":"elastic-apm-node"},"ecs":{"version":"1.6.0"},"message":"..."}

[basepi]

Yeah, python only does this with log.level right now, iirc.

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• Start Time: 2023-01-20T04:38:00.032+0000

• Duration: 6 min 30 sec

[apmmachine]

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• Start Time: 2023-01-23T04:38:00.031+0000

• Duration: 7 min 33 sec

[SylvainJuge]

Already using ecs.version as a top-level field in Java.

[Mpdreamz]

Thanks for the reviews! Merging now.