Upgrade briar with random test data for private groups

Draft since depends on briar!1788 (merged) to be merged.

Shouldn't fully close #113 (closed) since this does not add private groups for our deterministic test data.

briar-desktop/src/main/kotlin/org/briarproject/briar/desktop/navigation/BriarSidebar.kt

@@ -96,7 +96,7 @@ fun BriarSidebar(
         Spacer(Modifier.weight(1f))
 
         if (configuration.shouldEnableTransportSettings()) BriarSidebarButton(UiMode.TRANSPORTS)
-        if (configuration.shouldEnableMailbox()) BriarSidebarButton(
+        BriarSidebarButton(
             selected = uiMode == UiMode.MAILBOX,
             onClick = { setUiMode(UiMode.MAILBOX) },
             icon = UiMode.MAILBOX.icon,