Use SHA256 for temporary user code

[mpetrowi]

This PR changes the algorithm to generate temporary user codes from MD5 to SHA256. Temporary user codes seem to be used mainly by classic quizzes.

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
• logout from canvas and login again
• create a classic quiz and preview it
• submit the preview
• verify that the summary displays correctly

[CLAassistant]

All committers have signed the CLA.