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
@Override
        public XContentParser parser() {
            return this.parser;
        }
        @Override
        public LuceneDocument rootDoc() {
            return documents.get(0);
        }
        @Override
        public LuceneDocument doc() {
            return this.document;
        }
        @Override
        protected void addDoc(LuceneDocument doc) {
            numNestedDocs++;
            if (numNestedDocs > maxAllowedNumNestedDocs) {
                throw new DocumentParsingException(
                    parser.getTokenLocation(),
                    "The number of nested documents has exceeded the allowed limit of ["
                        + maxAllowedNumNestedDocs
                        + "]."
                        + " This limit can be set by changing the ["
                        + MapperService.INDEX_MAPPING_NESTED_DOCS_LIMIT_SETTING.getKey()
                        + "] index level setting."
                );
            }
            this.documents.add(doc);
        }
        @Override
        public Iterable<LuceneDocument> nonRootDocuments() {