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
public void testDynamicShadowingOfRuntimeSubfields() throws IOException {
        
        
        DocumentMapper mapper = createDocumentMapper(topMapping(b -> {
            b.startObject("runtime");
            b.startObject("obj").field("type", "test-composite").endObject();
            b.endObject();
        }));
        
        
        
        ParsedDocument doc = mapper.parse(source(b -> {
            b.startObject("obj");
            b.field("foo", "ignored");
            b.field("baz", "indexed");
            b.field("bar", "ignored");
            b.endObject();
        }));
        assertNull(doc.rootDoc().getField("obj.foo"));
        assertNotNull(doc.rootDoc().getField("obj.baz"));
        assertNull(doc.rootDoc().getField("obj.bar"));
        assertNotNull(doc.dynamicMappingsUpdate());
    }
    public void testDynamicFalseMatchesRoutingPath() throws IOException {
        DocumentMapper mapper = createMapperService(
            Settings.builder()
                .put(getIndexSettings())
                .put(IndexSettings.MODE.getKey(), "time_series")
                .put(IndexMetadata.INDEX_ROUTING_PATH.getKey(), "dim.*")
                .put(IndexSettings.TIME_SERIES_START_TIME.getKey(), "2021-04-28T00:00:00Z")