Use YAML.unsafe_load to allow alias in configuration

[negipo]

closes #1022
This PR suppress Psych::BadAlias error with configuration using aliases in Ruby 3.1.
YAML.unsafe_load is used because there can be no dangerous values in the configuration file.

[gblair]

I'm not sure, but wouldn't it be more appropriate to use:

YAML.load(processed, aliases: true)[::Rails.env]

[negipo]

It seems yes, but YAML.unsafe_load would have better backward compatibility.
Rails got same solution in rails/rails#42257