Remove support for type, fields, copy_to and boost in metadata field definition

[javanna]

Support for type, fields, copy_to and boost in metadata field definition has been deprecated in #90989. Such fields have been long parsed and silently ignored.

This commit removes support for their parsing.

[elasticsearchmachine]

Pinging @elastic/es-search-foundations (Team:Search Foundations)

[elasticsearchmachine]

Hi @javanna, I've created a changelog YAML for you. Note that since this PR is labelled >breaking, you need to update the changelog YAML to fill out the extended information sections.

[drempapis]

I tried to add the copy_to into _routing metadata field,

PUT /index
{
  "mappings": {
    "_routing": {
      "copy_to" : "any"
    },
    "properties": {
      "name": {
        "type": "text"
      }
    }
  }
}

Which adds a warning under the *_deprecation.json file (main branch)

Executing the same request and applying the PR changes, I get the response, which seems ok to me.

{
    "error": {
        "root_cause": [
            {
                "type": "mapper_parsing_exception",
                "reason": "unknown parameter [copy_to] on metadata field [_routing]"
            }
        ],
        "type": "mapper_parsing_exception",
        "reason": "Failed to parse mapping: unknown parameter [copy_to] on metadata field [_routing]",
        "caused_by": {
            "type": "mapper_parsing_exception",
            "reason": "unknown parameter [copy_to] on metadata field [_routing]"
        }
    },
    "status": 400
}

[javanna]

Thanks for the testing @drempapis and the review ! Indeed, these fields received special treatment before because they could potentially be provided, although without taking any effect. With my change, they get the same treatment as any other unknown field.