Currently the `index.look_ahead_time` index setting controls the `index.time_series_end_time` index setting when rolling over a tsdb data stream. The `index.look_ahead_time` setting default to 2 hours, which means the by default the `index.time_series_end_time` index setting of the new backing index will be 2 hours later than the `index.time_series.start_time` index setting. The the `index.time_series.start_time` and `index.time_series_end_time` index settings  control what `@timestamp` are allowed to be indexed.

Additionally the `index.look_ahead_time` setting is used to generate the `index.time_series.start_time` and `index.time_series_end_time` index settings when creating a new data stream for the first backing index. Based on the defaults of `index.look_ahead_time` setting, the  `index.time_series.start_time` index is set to current time - 2 hours and the `index.time_series_end_time` index setting is set to current time + 2 hours.

However for indexing the initial data setting the `index.time_series.start_time` index setting to 2 hours in the past sometimes creates too small time window. For example because data gets indexed starting from midnight that day. 

The idea is to add a `index.look_back_time` setting that controls how to generate the `index.time_series.start_time` index setting when creating the data stream (for the first backing index). This allows for creating a first backing index that accepts data that is earlier then current time - $index.look_ahead_time, without affecting how `index.time_series.start_time` and `index.time_series_end_time` index settings are generated during rollover.

I think a good default for the `index.look_back_time` setting would be 24 hours. Meaning that by default the `index.time_series.start_time` index setting for the first backing index of a new data stream would be current time - 24 hours.

Relates to elastic/integrations#7345
modules/data-streams/src/javaRestTest/java/org/elasticsearch/datastreams/TsdbDataStreamRestIT.java
Instant[] timestamps = new Instant[] {
                time,
                time.plusSeconds(1),
                time.plusSeconds(5),
                time.minus(30, ChronoUnit.DAYS),
                time.plus(30, ChronoUnit.DAYS) };
            for (Instant timestamp : timestamps) {
                var indexRequest = new Request("POST", "/k8s/_doc");
                indexRequest.setJsonEntity(DOC.replace("$time", formatInstant(timestamp)));
                var response = client().performRequest(indexRequest);
                assertOK(response);
                var responseBody = entityAsMap(response);
                assertThat((String) responseBody.get("_index"), backingIndexEqualTo("k8s", 2));
            }
        }
    }
    public void testUpdateComponentTemplateDoesNotFailIndexTemplateValidation() throws IOException {
        var request = new Request("POST", "/_component_template/custom_template");
        request.setJsonEntity("""
            {
                "template": {
                    "settings": {
                        "index": {
                            "number_of_replicas": 1
                        }
                    }
                }
            }
            """);
        client().performRequest(request);