Revert "bundler: 2.5.16 -> 2.5.21" (NixOS#353045)

This reverts commit 4dd40b1.

revert the bundler update to fix the gitlab nixosTest,
gitlab apparently wants only the old version.

Author: itepastra

pkgs/development/ruby-modules/bundler/default.nix

@@ -4,8 +4,8 @@ buildRubyGem rec {
   inherit ruby;
   name = "${gemName}-${version}";
   gemName = "bundler";
-  version = "2.5.21";
-  source.sha256 = "sha256-ID1kXl9tqmC7+0zMsafUq6gXFnTgC8KHJChHQPjZ7/s=";
+  version = "2.5.16";
+  source.sha256 = "sha256-h7oDOPQNWSinmxqwZ4Z6Ns3lgTXQi2MmfvY5eVuVmDo=";
   dontPatchShebangs = true;
 
   postFixup = ''