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
.put(IndexSettings.TIME_SERIES_END_TIME.getKey(), "2021-04-29T00:00:00Z")
                .build(),
            mapping(b -> {
                b.startObject("dim");
                b.field("type", "object");
                b.field("dynamic", false);
                b.endObject();
            })
        ).documentMapper();
        Exception e = expectThrows(DocumentParsingException.class, () -> mapper.parse(source(b -> {
            b.startObject("dim");
            b.field("foo", "bar");
            b.endObject();
        })));
        assertThat(
            e.getMessage(),
            equalTo("[1:15] All fields matching [routing_path] must be mapped but [dim.foo] was declared as [dynamic: false]")
        );
        e = expectThrows(DocumentParsingException.class, () -> mapper.parse(source(b -> {
            b.startObject("dim");
            b.startObject("foo").field("bar", "baz").endObject();
            b.endObject();
        })));
        assertThat(
            e.getMessage(),
            equalTo("[1:15] All fields matching [routing_path] must be mapped but [dim.foo] was declared as [dynamic: false]")
        );
    }
    public void testDocumentDescriptionInTsdb() throws IOException {
        DocumentMapper mapper = createMapperService(