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
Settings.builder()
                .put(getIndexSettings())
                .put(IndexSettings.MODE.getKey(), "time_series")
                .put(IndexMetadata.INDEX_ROUTING_PATH.getKey(), "uid")
                .put(IndexSettings.TIME_SERIES_START_TIME.getKey(), "2021-04-28T00:00:00Z")
                .put(IndexSettings.TIME_SERIES_END_TIME.getKey(), "2021-04-29T00:00:00Z")
                .build(),
            mapping(b -> {
                b.startObject("foo").field("type", "long").endObject();
                b.startObject("uid").field("type", "keyword").field("time_series_dimension", true).endObject();
            })
        ).documentMapper();
        {
            DocumentParsingException exception = expectThrows(
                DocumentParsingException.class,
                () -> mapper.parse(source(null, b -> b.field("foo", true), null))
            );
            assertThat(
                exception.getMessage(),
                containsString("failed to parse field [foo] of type [long] in a time series document. Preview of field's value: 'true'")
            );
        }
        {
            DocumentParsingException exception = expectThrows(
                DocumentParsingException.class,
                () -> mapper.parse(source(null, b -> b.field("@timestamp", "2021-04-28T00:01:00Z").field("foo", true), null))
            );
            assertThat(
                exception.getMessage(),
                equalTo(