upgrade mill to 0.11.0 (backport #3367) (#3407)

* Upgrade mill to 0.11.0 (#3367)

Also add mill to CI.

(cherry picked from commit 1f485a1a

)

# Conflicts:
#	build.sc

* Resolve backport conflicts

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Co-authored-by: Jiuyang Liu <liu@jiuyang.me>
Co-authored-by: Jack Koenig <koenig@sifive.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>