treewide: Change URLs to HTTPS

Lots of URLs were HTTP redirect to HTTPS. Changed those and checked them
if there's actual content. Inspired by
https://github.com/NixOS/nixpkgs/issues/60004

pkgs/applications/misc/doing/default.nix

@@ -17,7 +17,7 @@ bundlerEnv rec {
       in a TaskPaper-formatted text file. It allows for multiple
       sections/categories and flexible output formatting.
     '';
-    homepage    = http://brettterpstra.com/projects/doing/;
+    homepage    = https://brettterpstra.com/projects/doing/;
     license     = licenses.mit;
     maintainers = with maintainers; [ ktf ];
     platforms   = platforms.unix;