Use SHA256 as digest class

This is similar to what we did in commit 00a5dc921 when upgrading to
Rails 5.2. Quoting from that commit:

> Note this change will cause all fragment caching to expire. We
> consider it acceptable considering the page where caching is most
> important (stats) is barely affected by this change, since this change
> only affects the view, and the time-consuming operations are cached in
> the model.
>
> Comments are actually affected, though, and pages with thousands of
> comments might take a few extra seconds to load the first time they're
> accessed after this change. We don't think this is going to be an
> issue on existing CONSUL installations.

config/initializers/new_framework_defaults_7_0.rb

@@ -28,7 +28,7 @@ Rails.application.config.active_support.key_generator_hash_digest_class = OpenSS
 # Change the digest class for ActiveSupport::Digest.
 # Changing this default means that for example Etags change and
 # various cache keys leading to cache invalidation.
-# Rails.application.config.active_support.hash_digest_class = OpenSSL::Digest::SHA256
+Rails.application.config.active_support.hash_digest_class = OpenSSL::Digest::SHA256
 
 # Don't override ActiveSupport::TimeWithZone.name and use the default Ruby
 # implementation.