Sec: Update hackney to version 4.9.1

Hackney 4.9 enforces max_body while it reads and returns
{:error, {:closed, partial}} for a truncated body; the hackney
regression tests now pin that. The certifi override goes, as
hackney itself now asks for certifi ~> 2.18.0.

mix.exs

@@ -224,9 +224,7 @@ defmodule Pleroma.Mixfile do
       {:mix_audit, "~> 2.1", only: [:dev, :test], runtime: false},
       {:mock, "~> 0.3.9", only: :test},
       {:covertool, "~> 2.0", only: :test},
-      {:hackney, "~> 4.7.3", override: true},
-      # Hackney 4.7.3 pins certifi ~> 2.17.0; 2.18.0 drops the distrusted emSign roots.
-      {:certifi, "~> 2.18.0", override: true},
+      {:hackney, "~> 4.9.1", override: true},
       {:mox, "~> 1.2", only: :test},
       {:websockex, "~> 0.4.3", only: :test},
       {:benchee, "~> 1.4", only: :benchmark},