license: replace Shift Apache headers

[benma]

Replace the existing Shift Crypto/Shift Devices Apache-2.0 boilerplate header blocks with the standard SPDX License Identifier tag: "SPDX-License-Identifier: Apache-2.0".

This is better than the previous multi-line text header because it is machine-readable, unambiguous, and widely understood by tooling (scanners, compliance automation), while keeping files smaller and reducing header churn.