Post-release chores after having released Solidus v4.2.1

This code has been automatically generated by our 'Prepare post-release' GitHub
action.

Make sure that:

- [ ] The new release has been published, along with its tag. See https://github.com/solidusio/solidus/releases/tag/v4.2.1
- [ ] The corresponding patch branch exists. See https://github.com/solidusio/solidus/tree/v4.2
- [ ] The corresponding backport-v4.2 label exists. See https://github.com/solidusio/solidus/labels

core/lib/spree/core/version.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 module Spree
-  VERSION = "4.2.1"
+  VERSION = "4.2.2.dev"
 
   def self.solidus_version
     VERSION