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
assertNoSubobjects(doc);
    }
    public void testSubobjectsFalseFlatPaths() throws Exception {
        DocumentMapper mapper = createDocumentMapper(
            mapping(b -> b.startObject("metrics.service").field("type", "object").field("subobjects", false).endObject())
        );
        ParsedDocument doc = mapper.parse(source("""
            {
              "metrics.service.time" : 10,
              "metrics.service.time.max" : 500,
              "metrics.service.time.min" : 1,
              "metrics.service.test.with.dots" : "value",
              "metrics.object.inner.field" : "value"
            }
            """));
        assertNoSubobjects(doc);
    }
    public void testSubobjectsFalseMixedSubobjectPaths() throws Exception {
        DocumentMapper mapper = createDocumentMapper(
            mapping(b -> b.startObject("metrics.service").field("type", "object").field("subobjects", false).endObject())
        );
        ParsedDocument doc = mapper.parse(source("""
            {
              "metrics": {
                "service.time": 10,
                "service": {
                  "time.max" : 500,
                  "time.min" : 1
                },
                "object.inner.field" : "value"