mpop: libidn -> libidn2

as per the changelog entry for version 1.4.0 at
https://git.marlam.de/gitweb/?p=mpop.git;a=blob;f=NEWS;h=8b8f2fd45a2109ba04c9720de7973a519bc4aeaf;hb=HEAD,
libidn2 is used instead of libidn where IDN support is needed, so we
migrate to that.

pkgs/by-name/mp/mpop/package.nix

@@ -5,7 +5,7 @@
   gnutls,
   openssl,
   gsasl,
-  libidn,
+  libidn2,
   pkg-config,
   nlsSupport ? true,
   idnSupport ? true,
@@ -33,7 +33,7 @@ stdenv.mkDerivation (finalAttrs: {
 
   buildInputs =
     lib.optional gsaslSupport gsasl
-    ++ lib.optional idnSupport libidn
+    ++ lib.optional idnSupport libidn2
     ++ lib.optional (sslLibrary == "gnutls") gnutls
     ++ lib.optional (sslLibrary == "openssl") openssl;