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/test/java/org/elasticsearch/index/mapper/DocumentParserTests.java
DocumentMapper mapper = createDocumentMapper(
            mapping(b -> b.startObject("metrics").field("type", "object").field("subobjects", false).endObject())
        );
        DocumentParsingException err = expectThrows(DocumentParsingException.class, () -> mapper.parse(source("""
            {
              "metrics.service.time": [
                {
                  "max" : 1000
                }
              ]
            }
            """)));
        assertEquals("[3:5] Tried to add subobject [service.time] to object [metrics] which does not support subobjects", err.getMessage());
    }
    public void testSubobjectsFalseParseGeoPoint() throws Exception {
        DocumentMapper mapper = createDocumentMapper(mapping(b -> {
            b.startObject("metrics");
            {
                b.field("type", "object").field("subobjects", false);
                b.startObject("properties");
                {
                    b.startObject("location");
                    b.field("type", "geo_point");
                    b.endObject();
                }
                b.endObject();
            }
            b.endObject();
        }));
        ParsedDocument parsedDocument = mapper.parse(source("""