docs: release: Add note on encryption changes

- Reworked image encryption support for Zephyr, static dummy key files

  are no longer in the code, a pem file must be supplied to extract

  the private and public keys. The Kconfig menu has changed to only

  show a single option for enabling encryption and selecting the key

  file.

- Serial recovery can now read and handle encrypted seondary slot

  partitions.

- Serial recovery with MBEDTLS no longer has undefined operations which

  led to usage faults when the secondary slot image was encrypted.