As mentioned in
https://github.com/rails/rails/pull/40770#issuecomment-748347066 we
should default to SHA256 where SHA1 is used today. This switches over
the ActiveSupport::Digest to use SHA256 for new applications.
It also updates the constants to always refer to and use the OpenSSL
constants as well, as also discussed in that PR.
Using `truncate` to limit the length of the digest has the unwanted side
effect of adding an ellipsis when the input is longer than the limit.
Also:
- Don't instantiate a new object for every digest
- Rename the configuration option to `hash_digest_class`
- Update the CHANGELOG entry to describe how to use the feature
