Mark experimental fields as `beta` in generated files

[ymao1]

According to the ECS RFC stages doc, any RFC in stage 3 is considered GA. Any RFC in stage 2 can be commited as beta fields but are still considered experimental.

However there are many fields from stage 2 RFCs that are not marked as beta and there are fields from RFCs that have been approved and moved to stage 3 that are still marked as beta.

Here are the fields from RFCs in generated/ecs_flat.yml that are in stage 2 but not marked as beta:

https://github.com/elastic/ecs/blob/main/rfcs/text/0009-data_stream-fields.md
https://github.com/elastic/ecs/blob/main/rfcs/text/0015-create-file-elf.md
https://github.com/elastic/ecs/blob/main/rfcs/text/0027-faas-fields.md
https://github.com/elastic/ecs/blob/main/rfcs/text/0034-device-fields.md
https://github.com/elastic/ecs/blob/main/rfcs/text/0037-host-metrics.md - 2 fields added from this RFC, not marked as beta
https://github.com/elastic/ecs/blob/main/rfcs/text/0040-volume-device.md

In Kibana, we would like to exclude beta experimental fields from our ECS component template and so it would be great to correctly mark the stage 2 fields as beta (and remove the beta designation from GA fields)