Update dev dependency: postcss-import@^15.1.0 -> postcss-import@^16.1.0

[jesstelford]

The major change is just dropping old node support: https://github.com/postcss/postcss-import/blob/master/CHANGELOG.md#1600--2024-01-02

I confirmed the generated CSS was the same by running the below script on this branch, then on main, and doing a git diff --no-index .... (there was no difference):

if [ -z "$1" ]; then echo "A filename is required:\n> $0 ./postcss-import-15.css"; exit 1; fi;

# Write a temporary postcss config for cssnano to work
cat <<EOF > postcss.config.js
module.exports = {
  plugins: [
    require('cssnano')({
      preset: 'lite',
    }),
  ],
};
EOF

pnpm add --workspace-root cssnano cssnano-preset-lite
pnpm install
pnpm run build --filter="@shopify/polaris" --no-cache --force

cp polaris-react/build/esm/styles.css "$1" # Move out of ignored directories
pnpm prettier -w "$1" # Pre-format to normalize whitespace
pnpm dlx postcss-cli -r --no-map --config="`pwd`" "$1" # cssnano removes more whitespace prettier doesn't
pnpm prettier -w "$1" # Format again to get back into un-minified version
rm postcss.config.js # Clean up temporary files
pnpm remove --workspace-root cssnano cssnano-preset-lite

[chloerice]