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/test/java/org/elasticsearch/datastreams/LookAHeadTimeTests.java
@Override
    protected Collection<Class<? extends Plugin>> getPlugins() {
        return List.of(DataStreamsPlugin.class);
    }
    @After
    public void tearDown() throws Exception {
        updateClusterSettings(Settings.builder().put(DataStreamsPlugin.TIME_SERIES_POLL_INTERVAL.getKey(), (String) null).build());
        super.tearDown();
    }
    public void testTimeSeriesPollIntervalSetting() {
        var settings = Settings.builder().put(DataStreamsPlugin.TIME_SERIES_POLL_INTERVAL.getKey(), "9m").build();
        updateClusterSettings(settings);
    }
    public void testTimeSeriesPollIntervalSettingToLow() {
        var settings = Settings.builder().put(DataStreamsPlugin.TIME_SERIES_POLL_INTERVAL.getKey(), "1s").build();
        var e = expectThrows(IllegalArgumentException.class, () -> updateClusterSettings(settings));
        assertThat(e.getMessage(), equalTo("failed to parse value [1s] for setting [time_series.poll_interval], must be >= [1m]"));
    }
    public void testTimeSeriesPollIntervalSettingToHigh() {
        Settings settings = Settings.builder().put(DataStreamsPlugin.TIME_SERIES_POLL_INTERVAL.getKey(), "11m").build();
        var e = expectThrows(IllegalArgumentException.class, () -> updateClusterSettings(settings));
        assertThat(e.getMessage(), equalTo("failed to parse value [11m] for setting [time_series.poll_interval], must be <= [10m]"));
    }
    public void testLookAheadTimeSetting() throws IOException {
        var settings = Settings.builder().put(DataStreamsPlugin.LOOK_AHEAD_TIME.getKey(), "10m").build();