manifest: Update openthread to b21e99b.
manifest: Update openthread to b21e99b.

This commit updates openthread to b21e99b.
Secondly this commit aligns history of openthread to
Vanilla version 1:1.

Signed-off-by: Przemyslaw Bida <a50c93f369fde5722836c96336399b0b25c1b4ec@nordicsemi.no>

galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr