Use SHA256 for temporary ids in classic quizzes

[mpetrowi]

This PR changes the algorithm to generate temporary quiz ids from MD5 to SHA256.

This is one of a set of PRs to allow Canvas to run on a host with FIPS modules enabled. FIPS modules disable Digest::MD5 calls, and so we would like to switch to other more modern hashing algorithms.

Test plan:

• specs pass
• regression testing of classic quizzes, in particular:
  • fill in multiple blanks questions
  • multiple dropdowns questions

[CLAassistant]

All committers have signed the CLA.