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/version-management/gitless/default.nix

@@ -16,7 +16,7 @@ pythonPackages.buildPythonApplication rec {
   doCheck = false;
 
   meta = with stdenv.lib; {
-    homepage = http://gitless.com/;
+    homepage = https://gitless.com/;
     description = "A version control system built on top of Git";
     license = licenses.gpl2;
     platforms = platforms.all;