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
"test.with.dots" : {
                    "max" : 10
                  }
                }
              }
            }
            """)));
        assertEquals(
            "[4:26] Tried to add subobject [test.with.dots] to object [metrics.service] which does not support subobjects",
            err.getMessage()
        );
    }
    public void testSubobjectsFalseRootWithInnerObject() throws Exception {
        DocumentMapper mapper = createDocumentMapper(mappingNoSubobjects(xContentBuilder -> {}));
        DocumentParsingException err = expectThrows(DocumentParsingException.class, () -> mapper.parse(source("""
            {
              "metrics": {
                "service": {
                  "time.max" : 10
                }
              }
            }
            """)));
        assertEquals("[2:14] Tried to add subobject [metrics] to object [_doc] which does not support subobjects", err.getMessage());
    }
    public void testSubobjectsFalseRoot() throws Exception {
        DocumentMapper mapper = createDocumentMapper(mappingNoSubobjects(xContentBuilder -> {}));
        ParsedDocument doc = mapper.parse(source("""
            {
              "metrics.service.time" : 10,