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/chef-server/src/oc-id/app/models/signature.rb:17:5: W: Lint/DeprecatedOpenSSLConstant: Use OpenSSL::Digest.hexdigest('SHA1', canonical_string) instead of OpenSSL::Digest::SHA1.hexdigest(canonical_string).
    OpenSSL::Digest::SHA1.hexdigest(canonical_string)
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
/Users/tsmith/dev/work/chef-server/src/oc_erchef/apps/oc_chef_wm/test/password_gen.rb:13:9: W: Lint/DeprecatedOpenSSLConstant: Use OpenSSL::Digest.hexdigest('SHA1', "--#{salt}--#{password}--") instead of OpenSSL::Digest::SHA1.hexdigest("--#{salt}--#{password}--").
        OpenSSL::Digest::SHA1.hexdigest("--#{salt}--#{password}--")
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
/Users/tsmith/dev/work/chef-server/src/oc_erchef/apps/oc_chef_wm/test/password_gen.rb:15:9: W: Lint/DeprecatedOpenSSLConstant: Use OpenSSL::Digest.hexdigest('SHA1', "#{salt}--#{password}--") instead of OpenSSL::Digest::SHA1.hexdigest("#{salt}--#{password}--").
        OpenSSL::Digest::SHA1.hexdigest("#{salt}--#{password}--")
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Aha! Link: https://chef.aha.io/features/SH-3048