Currently, when an object is mapped with `subobjects: false`, trying to ingest a document that has nested objects leads to a mapping exception.

In a couple of occasions, we've been talking about that it would be convenient if Elasticsearch could automatically flatten the fields instead of the client having to know about the `subobjects` setting. This would make `subobjects: false` more of a drop-in replacement that allows for a much easier adoption, without changes in the client.

Note that the `_source` should ideally stay untouched, just the stored fields should be flattened automatically.

Some related issues
* https://github.com/elastic/apm/issues/347
* https://github.com/elastic/elasticsearch/issues/88777
* https://github.com/elastic/package-spec/issues/349
server/src/main/java/org/elasticsearch/index/mapper/DocumentParser.java
throw new StrictDynamicMappingException(context.parser().getTokenLocation(), context.parent().fullPath(), lastFieldName);
            } else if (context.dynamic() == ObjectMapper.Dynamic.FALSE) {
                context.parser().skipChildren();
            } else {
                Mapper objectMapperFromTemplate = DynamicFieldsBuilder.createObjectMapperFromTemplate(context, lastFieldName);
                if (objectMapperFromTemplate == null) {
                    parseNonDynamicArray(context, lastFieldName, lastFieldName);
                } else {
                    if (parsesArrayValue(objectMapperFromTemplate)) {
                        context.addDynamicMapper(objectMapperFromTemplate);
                        context.path().add(lastFieldName);
                        parseObjectOrField(context, objectMapperFromTemplate);
                        context.path().remove();
                    } else {
                        parseNonDynamicArray(context, lastFieldName, lastFieldName);
                    }
                }
            }
        }
    }
    private static boolean parsesArrayValue(Mapper mapper) {
        return mapper instanceof FieldMapper && ((FieldMapper) mapper).parsesArrayValue();
    }
    private static void parseNonDynamicArray(DocumentParserContext context, final String lastFieldName, String arrayFieldName)
        throws IOException {
        XContentParser parser = context.parser();
        XContentParser.Token token;
        while ((token = parser.nextToken()) != XContentParser.Token.END_ARRAY) {
            if (token == XContentParser.Token.START_OBJECT) {
                parseObject(context, lastFieldName);