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/development/tools/redis-dump/default.nix

@@ -9,6 +9,7 @@ bundlerEnv {
   buildInputs = [ perl autoconf ];
 
   meta = with lib; {
+    broken = true; # needs ruby 2.0
     description = "Backup and restore your Redis data to and from JSON";
     homepage    = http://delanotes.com/redis-dump/;
     license     = licenses.mit;