Use GHC 8.8 as the minimum bootstrap compiler version
This patch makes GHC 8.8 the minimum bootstrap compiler across all Docker images in preparation for the same requirement being imposed on GHC. In most cases, I was able to use 8.8.3. The exception is `x86_64-linux-deb10`, where the only 8.8 bindist I could find was 8.8.1.
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