refactor(deps): use conflate instead of merge crate

[simonsan]

Needs rustic-rs/rustic_core#284 to be merged and released first.

Fixes #1226

[simonsan]

/// Merge backup sources
///
/// If a source is already defined on left, use that. Else add it.
///
/// # Arguments
///
/// * `left` - Vector of backup sources
pub(crate) fn merge_sources(left: &mut Vec<BackupCmd>, mut right: Vec<BackupCmd>) {
    left.append(&mut right);
    left.sort_by(|opt1, opt2| opt1.sources.cmp(&opt2.sources));
    left.dedup_by(|opt1, opt2| opt1.sources == opt2.sources);
}

This could be a strategy, we could make more generic and move to the conflate crate. The question is though, if a HashMap/BTreeMap is better here instead, because we also deduplicate like that. Is there another reason to sort it?