Avoid defining Hash#deep_merge and #deep_merge!

`config` uses `DeepMerge.deep_merge!` instead of `Hash#deep_merge!`, so
monkey patching `Hash` is unnecessary.  Furthermore, DeepMerge's `Hash`
monkey patch is not compatible with Rails 7.1 (see [rails#49457][]).

This commit changes `require 'deep_merge'` to `require 'deep_merge/core'`
so that DeepMerge's `Hash` monkey patch is no longer loaded.  Users who
rely the monkey patch can load it manually via
`require 'deep_merge/deep_merge_hash'`.

Closes rubyconfig#314.

[rails#49457]: rails/rails#49457

lib/config.rb

@@ -6,7 +6,7 @@
 require 'config/sources/hash_source'
 require 'config/sources/env_source'
 require 'config/validation/schema'
-require 'deep_merge'
+require 'deep_merge/core'
 
 module Config
   extend Config::Validation::Schema