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As a prerequisite to upgrading to Zephyr v2.7.0, upgrade CI to use
Zephyr docker image v0.21.0. In particular, this is needed to pick up a
newer CMake version because Zephyr v2.7.0 increased the minimum CMake
version required to 3.20.0.
Signed-off-by: Maureen Helm <maureen.helm@intel.com>