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
{
              "metrics": {
                "location" : {
                  "lat": 41.12,
                  "lon": -71.34
                }
              }
            }
            """));
        assertNotNull(parsedDocument.rootDoc().getField("metrics.location"));
        assertNull(parsedDocument.dynamicMappingsUpdate());
    }
    public void testSubobjectsFalseParentDynamicFalse() throws Exception {
        
        
        DocumentMapper mapper = createDocumentMapper(mapping(b -> {
            b.startObject("metrics");
            {
                b.field("dynamic", false);
                b.startObject("properties");
                {
                    b.startObject("service");
                    b.field("type", "object").field("subobjects", false);
                    b.endObject();
                }
                b.endObject();
            }
            b.endObject();
        }));
        ParsedDocument doc = mapper.parse(source("""