ruby: remove unsupported versions (#34927)

* remove EOL ruby versions for security and maintenance reasons.
* only expose ruby_MAJOR_MINOR to the top-level. we don't provide
guarantees for the TINY version.
* mark all related packages as broken
* switch the default ruby version from 2.3.x to 2.4.x

pkgs/tools/backup/backup/default.nix

@@ -1,14 +1,15 @@
-{ stdenv, lib, bundlerEnv, ruby_2_1, curl }:
+{ stdenv, lib, bundlerEnv, ruby, curl }:
 
 bundlerEnv {
   name = "backup_v4";
 
-  ruby = ruby_2_1;
+  inherit ruby;
   gemdir = ./.;
 
   buildInputs = [ curl ];
 
   meta = with lib; {
+    broken = true; # need ruby 2.1
     description = "Easy full stack backup operations on UNIX-like systems";
     homepage    = http://backup.github.io/backup/v4/;
     license     = licenses.mit;