Remove deprecated non-symbol access to nested config_for hashes

[etiennebarrie]

Summary

This fixes an issue with #37870 where nested hashes returned by config_for could return Hash subclasses instead of strictly a Hash when calling to_hash.

Since we changed the name of the for option in HashWithIndifferentAccess, and the class dealing with deprecations takes an options hash, there's no exception but the option is silently ignored, which in turn changes the behavior of HashWithIndifferentAccess#to_hash.

Instead of fixing the code, this removes the deprecation code, which also solves the issue.

# before #37870 
Rails.application.config_for(:example)[:a_hash].to_hash[:other_hash]['key'] # => nil

# after #37870 
Rails.application.config_for(:example)[:a_hash].to_hash[:other_hash]['key']
# DEPRECATION WARNING: Accessing hashes returned from config_for by non-symbol keys is deprecated and will be removed in Rails 6.1. Use symbols for access instead. (called from <main> at (pry):1)
# => "value"

# after this PR
Rails.application.config_for(:example)[:a_hash].to_hash[:other_hash]['key'] # => nil

Other Information

I used the symbolize_names option YAML.load, which seems to be the first usage of it in Rails. This required me to symbolize the environment name, and the "shared" special key.
Another possibility would be to use deep_symbolize_keys as was done before #35198, but since we're loading the configuration file ourselves, it felt like an unnecessary traversal of the whole data structure. Let me know if that was a mistake.

@rafaelfranca
cc @byroot @Edouard-chin @paracycle @Morriar

[rafaelfranca]

Can you add a CHANGELOG entry? We also need a entry in the upgrading guide since this is a deprecation removal.

[etiennebarrie]

Not sure how verbose I needed to be. 😬