#1494 broke build due to API change in `css-loader`

[alphapapa]

The dependencies update in #1494 broke the building of the theme. The errors are due to an API change in one of the modules: https://github.com/pydata/pydata-sphinx-theme/actions/runs/6418399221/job/17426117223#step:4:917

throw new Error("Module build failed (from ./node_modules/css-loader/dist/cjs.js):\nValidationError: Invalid options object. CSS Loader has been initialized using an options object that does not match the API schema.\n - options has an unknown property '-url'. These properties are valid:\n   object { url?, import?, modules?, sourceMap?, importLoaders?, esModule?, exportType? }\n    at validate (/home/runner/work/pydata-sphinx-theme/pydata-sphinx-theme/node_modules/webpack/node_modules/schema-utils/dist/validate.js:105:11)\n    at Object.getOptions (/home/runner/work/pydata-sphinx-theme/pydata-sphinx-theme/node_modules/webpack/lib/NormalModule.js:587:19)\n    at Object.loader (/home/runner/work/pydata-sphinx-theme/pydata-sphinx-theme/node_modules/css-loader/dist/index.js:20:27)");

The update to css-loader in that PR moved it several versions ahead:

Updates `css-loader` from 3.6.0 to 6.8.1

I'll submit a PR that fixes the build in a moment, but it will need careful review, because I'm not sure if it's the correct fix.

@jarrodmillman