[ci skip] doc: updated the default digest algorithm used for key derivation in ActiveRecord Encryption to SHA256.

https://guides.rubyonrails.org/upgrading_ruby_on_rails.html#active-record-encryption-algorithm-changes

```ruby
irb(main):001> Rails.configuration.active_record.encryption.hash_digest_class
=> OpenSSL::Digest::SHA256
irb(main):002> ActiveRecord::Encryption.config.hash_digest_class
=> OpenSSL::Digest::SHA256
```

Author: rhiroe

guides/source/active_record_encryption.md

@@ -490,7 +490,7 @@ The default encoding for attributes encrypted deterministically. You can disable
 
 #### `config.active_record.encryption.hash_digest_class`
 
-The digest algorithm used to derive keys. `OpenSSL::Digest::SHA1` by default.
+The digest algorithm used to derive keys. `OpenSSL::Digest::SHA256` by default.
 
 #### `config.active_record.encryption.support_sha1_for_non_deterministic_encryption`