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 testSubobjectsFalseWithInnerObject() throws Exception {
        DocumentMapper mapper = createDocumentMapper(
            mapping(b -> b.startObject("metrics.service").field("type", "object").field("subobjects", false).endObject())
        );
        DocumentParsingException err = expectThrows(DocumentParsingException.class, () -> mapper.parse(source("""
            {
              "metrics": {
                "service": {
                  "time" : {
                    "max" : 10
                  }
                }
              }
            }
            """)));
        assertEquals(
            "[4:16] Tried to add subobject [time] to object [metrics.service] which does not support subobjects",
            err.getMessage()
        );
    }
    public void testSubobjectsFalseWithInnerDottedObject() throws Exception {
        DocumentMapper mapper = createDocumentMapper(
            mapping(b -> b.startObject("metrics.service").field("type", "object").field("subobjects", false).endObject())
        );
        DocumentParsingException err = expectThrows(DocumentParsingException.class, () -> mapper.parse(source("""
            {
              "metrics": {
                "service": {