Fix default value of ZO_INGEST_FLATTEN_LEVEL

[IcarusSosie]

According to the docs, ZO_INGEST_FLATTEN_LEVEL's default value is 5, but the default value in code is 3 :

    // [...]
    pub query_group_base_speed: usize,
    #[env_config(name = "ZO_INGEST_ALLOWED_UPTO", default = 5)] // in hours - in past
    pub ingest_allowed_upto: i64,
    #[env_config(name = "ZO_INGEST_FLATTEN_LEVEL", default = 3)] // default flatten level
    pub ingest_flatten_level: u32,
    #[env_config(name = "ZO_IGNORE_FILE_RETENTION_BY_STREAM", default = false)]
    pub ignore_file_retention_by_stream: bool,
    // [...]

This could lead to dataloss if a developer considers 5 to be enough according to the depth of their logs, but 3 wouldn't be.

Not totally unrelated, a list of BC breaks on release pages would be quite great 🥲.

[hengfeiyang]

Thank you