Switch on validation of encrypted ciphertexts

[eversC]

...right now this would only apply to keys stored in GitHub, as that's the only time encryption is needed.

ovotech/mantle has the ability to validate ciphertexts for us, so we'd just need to switch it on here:

cloud-key-rotator/pkg/crypt/crypt.go

Line 30 in b1aae8e

const disableValidation = true

I can't remember the original reason why this was disabled, so thorough testing required.