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/main/java/org/elasticsearch/datastreams/DataStreamIndexSettingsProvider.java
private final CheckedFunction<IndexMetadata, MapperService, IOException> mapperServiceFactory;
    DataStreamIndexSettingsProvider(CheckedFunction<IndexMetadata, MapperService, IOException> mapperServiceFactory) {
        this.mapperServiceFactory = mapperServiceFactory;
    }
    @Override
    public Settings getAdditionalIndexSettings(
        String indexName,
        String dataStreamName,
        boolean timeSeries,
        Metadata metadata,
        Instant resolvedAt,
        Settings allSettings,
        List<CompressedXContent> combinedTemplateMappings
    ) {
        if (dataStreamName != null) {
            DataStream dataStream = metadata.dataStreams().get(dataStreamName);
            
            
            
            boolean migrating = dataStream != null
                && (dataStream.getIndexMode() == null || dataStream.getIndexMode() == IndexMode.STANDARD)
                && timeSeries;
            IndexMode indexMode;
            if (migrating) {
                indexMode = IndexMode.TIME_SERIES;
            } else if (dataStream != null) {
                indexMode = timeSeries ? dataStream.getIndexMode() : null;
            } else if (timeSeries) {
                indexMode = IndexMode.TIME_SERIES;
            } else {