Previously, wrapped DEKs written to `data_encryption_key.json` were
encoded using `Base64.encode64`, which inserted a newline every 60
characters and always appended a trailing newline. Later, this was
changed to `Base64.strict_encode64`, which produces a single
uninterrupted string.
While `Base64.decode64` can handle both formats,
`Base64.strict_decode64` fails if newlines are present.
Since `StorageKeyEncryption.read_encrypted_dek` is called when starting
old VMs after a server reboot, it must remain backward compatible.
This change updates `StorageKeyEncryption.read_encrypted_dek` to use
`Base64.decode64`.
4c6f74738b