Remove ecs.version field from mappings

[axw]

Motivation/summary

Remove the ecs.version field mapping from APM data streams. We don't use this field at all in APM UI, and its presence works against converging on OTel SemConv, and decoupling the index templates from apm-server.

Checklist

• Update CHANGELOG.asciidoc
• Update package changelog.yml (only if changes to apmpackage have been made)
  - [ ] Documentation has been updated

How to test these changes

1. Ingest some APM data (e.g. with apmtool)
2. Verify the ecs.version field is not present in any APM docs
3. Verify basic APM UI functionality still works

Related issues

elastic/elasticsearch#97546

[mergify]

This pull request does not have a backport label. Could you fix it @axw? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-7.17 is the label to automatically backport to the 7.17 branch.
• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.

NOTE: backport-skip has been added to this pull request.

[lahsivjar]

Out of curiosity, even the previous ecsVersionPipeline was removing the field ecs.version. Does this mean we were never indexing ecs.version so it was not used in the mappings anyway?

[axw]

The ingest pipeline was there to remove the field from documents written by older versions of apm-server. The field would get added back into documents as a constant_keyword field.

I've left the removal there for now, but I don't think we'll keep that in elastic/elasticsearch#97546. We'll just dynamically map the field for older versions of apm-server - no big deal.

[lahsivjar]

Tested successfully with BC3 that ecs.version field is not present in any APM docs.

^ All docs with `ecs.version` not present. ^ All docs with `ecs.version` present.