Revert "[CMake] Bumps minimum version to 3.20.0."

Some build bots have not been updated to the new minimal CMake version.
Reverting for now and ping the buildbot owners.

This reverts commit 44c6b905f8527635e49bb3ea97dea315f92d38ec.

GitOrigin-RevId: 92523a35a827539db8557bbc3ecab7f9ea3f6ade
1 file changed