newsboat: use $XDG_CONFIG_HOME/newsboat

As of Newsboat 2.19, configuration using XDG_CONFIG_HOME/newsboat is
supported:
https://newsboat.org/releases/2.19/docs/newsboat.html#_xdg_base_directory_support

This is a continuation of #1331 and includes gating logic so that the
change doesn't happen until state version 21.05.

Supersedes and Closes #1331

tests/modules/programs/newsboat/default.nix

@@ -1,4 +1,5 @@
 {
   newsboat-basics = ./newsboat-basics.nix;
   newsboat-basics-2003 = ./newsboat-basics-2003.nix;
+  newsboat-basics-2105 = ./newsboat-basics-2105.nix;
 }

tests/modules/programs/newsboat/newsboat-basics-2105.nix

@@ -0,0 +1,36 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+{
+  config = {
+    home.stateVersion = "21.05";
+
+    programs.newsboat = {
+      enable = true;
+
+      urls = [
+        {
+          url = "http://example.org/feed.xml";
+          tags = [ "tag1" "tag2" ];
+          title = "Cool feed";
+        }
+
+        { url = "http://example.org/feed2.xml"; }
+      ];
+
+      queries = { "foo" = ''rssurl =~ "example.com"''; };
+    };
+
+    nixpkgs.overlays = [
+      (self: super: { newsboat = pkgs.writeScriptBin "dummy-newsboat" ""; })
+    ];
+
+    # The format didn't change since 20.03, just the location.
+    nmt.script = ''
+      assertFileContent \
+        home-files/.config/newsboat/urls \
+        ${./newsboat-basics-urls-2003.txt}
+    '';
+  };
+}