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
assertOK(client().performRequest(putComposableIndexTemplateRequest));
        var rolloverRequest = new Request("POST", "/k8s/_rollover");
        var rolloverResponse = client().performRequest(rolloverRequest);
        assertOK(rolloverResponse);
        var rolloverResponseBody = entityAsMap(rolloverResponse);
        assertThat(rolloverResponseBody.get("rolled_over"), is(true));
        var newIndex = (String) rolloverResponseBody.get("new_index");
        assertThat(newIndex, backingIndexEqualTo("k8s", 6));
        
        var t = currentTime;
        for (int i = 0; i < numDocs; i++) {
            var indexRequest = new Request("POST", "/k8s/_doc");
            indexRequest.setJsonEntity(DOC.replace("$time", formatInstant(t)));
            var response = client().performRequest(indexRequest);
            assertOK(response);
            var responseBody = entityAsMap(response);
            assertThat((String) responseBody.get("_index"), backingIndexEqualTo("k8s", 6));
            t = t.plusMillis(1000);
        }
        
        var indexRequest = new Request("POST", "/k8s/_doc");
        indexRequest.setJsonEntity(DOC.replace("$time", formatInstant(currentMinus30Days)));
        var e = expectThrows(ResponseException.class, () -> client().performRequest(indexRequest));
        assertThat(e.getMessage(), containsString("is outside of ranges of currently writable indices"));
    }
    public void testDowngradeTsdbDataStreamToRegularDataStream() throws Exception {
        var time = Instant.now();
        {
            var indexRequest = new Request("POST", "/k8s/_doc");
            indexRequest.setJsonEntity(DOC.replace("$time", formatInstant(time)));