Switch ActiveSupport::HashDigest to SHA256

What does this MR do and why?

MD5 is not supported in FIPS mode

Removes the feature flag, and enables the switch starting in GitLab 15.2

https://gitlab.com/gitlab-org/gitlab/-/issues/220878

See also https://docs.gitlab.com/ee/update/#1520

Screenshots or screen recordings

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[2] pry(main)> plaintext = 'something that is plaintext'
=> "something that is plaintext"
[3] pry(main)> ActiveSupport::Digest.hexdigest(plaintext)
=> "043fa56f6ff062330e3cb5361282240c"
[4] pry(main)> OpenSSL::Digest::SHA256.hexdigest(plaintext)
=> "043fa56f6ff062330e3cb5361282240c703d08ba3cc3c5a1e2c35b6b76d8e7a9"

How to set up and validate locally

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MR acceptance checklist

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