Simplify TLS opts

- `verify_fun` is not useful now
- use `customize_check_hostname` (OTP 20+ so OK)
- `partial_chain` is useless as of OTP 21.1 (wasn't there, but hackney/..
uses it)

lib/pleroma/gun/conn.ex

@@ -28,9 +28,8 @@ defmodule Pleroma.Gun.Conn do
       cacertfile: CAStore.file_path(),
       depth: 20,
       reuse_sessions: false,
-      verify_fun:
-        {&:ssl_verify_hostname.verify_fun/3,
-         [check_hostname: Pleroma.HTTP.AdapterHelper.format_host(host)]}
+      log_level: :warning,
+      customize_hostname_check: [match_fun: :public_key.pkix_verify_hostname_match_fun(:https)]
     ]
 
     tls_opts =

lib/pleroma/http/adapter_helper/gun.ex

@@ -39,36 +39,8 @@ defmodule Pleroma.HTTP.AdapterHelper.Gun do
   defp add_scheme_opts(opts, %{scheme: "https"}) do
     opts
     |> Keyword.put(:certificates_verification, true)
-    |> Keyword.put(:tls_opts,
-      log_level: :warning,
-      customize_hostname_check: [match_fun: &ssl_match_fun/2]
-    )
   end
 
-  # ssl_match_fun is adapted from [Mint](https://github.com/elixir-mint/mint)
-  # Copyright 2018 Eric Meadows-Jönsson and Andrea Leopardi
-
-  # Wildcard domain handling for DNS ID entries in the subjectAltName X.509
-  # extension. Note that this is a subset of the wildcard patterns implemented
-  # by OTP when matching against the subject CN attribute, but this is the only
-  # wildcard usage defined by the CA/Browser Forum's Baseline Requirements, and
-  # therefore the only pattern used in commercially issued certificates.
-  defp ssl_match_fun({:dns_id, reference}, {:dNSName, [?*, ?. | presented]}) do
-    case domain_without_host(reference) do
-      '' ->
-        :default
-
-      domain ->
-        :string.casefold(domain) == :string.casefold(presented)
-    end
-  end
-
-  defp ssl_match_fun(_reference, _presented), do: :default
-
-  defp domain_without_host([]), do: []
-  defp domain_without_host([?. | domain]), do: domain
-  defp domain_without_host([_ | more]), do: domain_without_host(more)
-
   @spec get_conn(URI.t(), keyword()) :: {:ok, keyword()} | {:error, atom()}
   def get_conn(uri, opts) do
     case ConnectionPool.get_conn(uri, opts) do