Use SHA256 for cache keys

[mpetrowi]

This PR updates cache key computation to use SHA256 instead of MD5.

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.

This change should not affect site operation, so the running the specs should be a sufficient test.

Test plan:

• specs pass

[CLAassistant]

All committers have signed the CLA.