chore: bump staging to v2.8.0-rc.6 (#776)

Author: v0idpwn

deploy/upgrade/staging.config

@@ -1,7 +1,7 @@
 [
     {upgrade_from, "2.7.1"},
     {upgrade_previous, "2.7.3"},
-    {upgrade_to, "2.8.0-rc.4"},
+    {upgrade_to, "2.8.0-rc.6"},
     % list() | [<<"all">>]
     {regions, [<<"all">>]},
     % :soft | :hard