Beats lowercases Elasticsearch ingest pipeline names

Packetbeat version: `packetbeat-7.13.2-amd64.deb`

Hi team,
we are using an Ingest Pipeline for our DGA project.
The name of the pipeline is `ML_DGA_ingest_pipeline`.
We add the name of this pipeline in the config stanza in `packetbeat.yml` like this

```
output.elasticsearch:
hosts: ["your-hostname:your-port"]
pipeline: pipeline ML_DGA_ingest_pipeline
```

When we try to run packetbeat to ingest documents through this pipeline, packetbeat logs report an error `pipeline ml_dga_ingest_pipeline does not exist` - note that the pipeline name has been lowercased.
Note that Elasticsearch itself treats `ML_DGA_ingest_pipeline` and `ml_dga_ingest_pipeline` as two separate entities.