docs: release: Add note on encryption changes

Adds notes on fixes and changes related to encrypted image handling

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>

docs/release-notes.d/zephyr-encryption.md

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+- 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.