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This will ensure some minimal amount of testing while we continue
cutting releases off of M80.

This restores the versioned/milestones/m80 directory to the state before
the M82 branch day, but changes lucicfg variables so that the M80
builders are defined in the bucket chromium-m80 instead of
chromium-stable, which is still associated with M81.

Bug: 1062340
Change-Id: I38074ea90a67f18f7adc35735086280fc0bd89b6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2108124
Commit-Queue: Garrett Beaty <gbeaty@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>
Cr-Commit-Position: refs/heads/master@{#751436}