Maintenance: Update dependency doorkeeper to v5.6.0

Gemfile.lock

@@ -165,7 +165,7 @@ GEM
     diffy (3.4.2)
     domain_name (0.5.20190701)
       unf (>= 0.0.5, < 1.0.0)
-    doorkeeper (5.5.4)
+    doorkeeper (5.6.0)
       railties (>= 5)
     dotenv (2.8.1)
     dry-cli (0.7.0)