OpenSSL usage deprecation warnings from Rubocop

[tas50]

Rubocop has a new cop that detects an upcoming deprecation to the OpenSSL gem that's built into Ruby.

The openssl introducing the deprecation:
ruby/openssl#366

The new rubocop rule currently only in master:
rubocop/rubocop#7950

/Users/tsmith/dev/work/supermarket/src/supermarket/app/models/user.rb:229:5: W: Lint/DeprecatedOpenSSLConstant: Use OpenSSL::Digest.hexdigest('MD5', key_in_der_format) instead of OpenSSL::Digest::MD5.hexdigest(key_in_der_format).
    OpenSSL::Digest::MD5.hexdigest(key_in_der_format).scan(/../).join(':')
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^