v0.21.0

Breaking Changes

• The function for subscribing to changes to the OTA manifest has
  been renamed from golioth_ota_observe_manifest_async() to
  golioth_ota_manifest_subscribe(). This name change reflects the
  new subscription behavior described below. Applications using
  Golioth's reference firmware update implementation do not require
  any changes.
• Applications which use Golioth's sample library for storing WiFi
  credentials will need to maintain that functionality themselves
  going forward or migrate to Zephyr's WiFi Credentials library
  (see below).

Added

• The OTA service now periodically polls for updates to the OTA
  manifest, in addition to using CoAP observations to receive
  asynchronous notifications of changes to the manifest. The
  polling period defaults to 1 day and is configurable using
  CONFIG_GOLIOTH_OTA_MANIFEST_SUBSCRITION_POLL_INTERVAL_S.

Changed

• Zephyr samples use Zephyr's native WiFi Credentials library for
  storing WiFi network information, instead of the previous custom
  solution. This provides greater flexibility in the selection of
  WiFi security and storage backend options.
• Zephyr support has been upgraded to v4.2.1.
• nRF Connect SDK support has been upgraded to v3.1.1.