chore: update sharp and image minimizer plugin (#665)

The old version of share has intermittent installation issues, see
lovell/sharp#3851 .
These issues are fixed in the latest version, so we should update
here. Also update image-minimizer-webpack-plugin to avoid
incompatibility issues, because that uses sharp, and the latest version
appears to be tested with the latest sharp version.

Fixes: #664
Private-ref: https://tasks.opencraft.com/browse/BB-9953

Author: samuelallan72