nixos/acme: don't use --reuse-key

Reusing the same private/public key on renewal has two issues:

 - some providers don't accept to sign the same public key
   again (Buypass Go SSL)

 - keeping the same private key forever partly defeats the purpose of
   renewing the certificate often

Therefore, let's remove this option. People wanting to keep the same
key can set extraLegoRenewFlags to `[ --reuse-key ]` to keep the
previous behavior. Alternatively, we could put this as an option whose
default value is true.

nixos/doc/manual/release-notes/rl-2105.xml

@@ -804,6 +804,16 @@ environment.systemPackages = [
      the deprecated <option>services.radicale.config</option> is used.
     </para>
    </listitem>
+   <listitem>
+    <para>
+     In the <option>security.acme</option> module, use of <literal>--reuse-key</literal>
+     parameter  for Lego has been removed. It was introduced for HKPK, but this security
+     feature is now deprecated. It is a better security practice to rotate key pairs
+     instead of always keeping the same. If you need to keep this parameter, you can add
+     it back using <literal>extraLegoRenewFlags</literal> as an option for the
+     appropriate certificate.
+    </para>
+   </listitem>
   </itemizedlist>
  </section>
 

nixos/modules/security/acme.nix

@@ -152,7 +152,7 @@ let
     );
     renewOpts = escapeShellArgs (
       commonOpts
-      ++ [ "renew" "--reuse-key" ]
+      ++ [ "renew" ]
       ++ optionals data.ocspMustStaple [ "--must-staple" ]
       ++ data.extraLegoRenewFlags
     );