Remove salt field from EncryptedData

[dmjb]

Relates to #3317

Previously EncryptedData had a dedicated salt field. As part of investigating AES-256-GCM, I have decided to follow the Go crypto library's convention of prepending the nonce/salt to the ciphertext. As a result, the salt field is no longer needed.

Since we are still using the hardcoded salt at this time, this will not impact any data in production.

Also tweaked some unit tests to use the new encrypted data structure instead of relying on the old DB columns which will be deleted in a future PR.

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Fixes #(related issue)

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[stacklokbot]

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[coveralls]

coverage: 50.267% (+0.008%) from 50.259%
when pulling a97de23 on use-fixed-salt
into c0286cd on main.