[v3] Support sass-loader v8

[haoqunjiang]

The warning on the loader version was removed to avoid frustrations.
Users could postpone the sass-loader upgrade until they decide to move to Vue CLI v4.

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Underlying tools
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

Other information:

[ghulamhussain]

@sodatea This has created a new bug in v 3.12 when adding vuetify plugin using vue add vetify
`conflicting versions for project dependency "sass-loader":

• ^8.0.0 injected by generator "undefined"
• ^7.1.0 injected by generator "vue-cli-plugin-vuetify"

Using newer version (^8.0.0), but this may cause build errors.
`

[haoqunjiang]

It's a friendly warning but not a bug. The undefined message could be improved though.

[ghulamhussain]

i am having this error in when i added vuetify

Failed to compile.

./node_modules/vuetify/src/styles/components/_selection-controls.sass (./node_modules/css-loader??ref--9-oneOf-3-1!./node_modules/postcss-loader/src??ref--9-oneOf-3-2!./node_modules/sass-loader/dist/cjs.js??ref--9-oneOf-3-3!./node_modules/vuetify/src/styles/components/_selection-controls.sass)
Module build failed (from ./node_modules/sass-loader/dist/cjs.js):
ValidationError: Invalid options object. Sass Loader has been initialised using an options object that does not match the API schema.

• options has an unknown property 'data'. These properties are valid:
  object { implementation?, sassOptions?, prependData?, sourceMap?, webpackImporter? }
  at validate (/home/hussaghu/Practice/vuetest/node_modules/sass-loader/node_modules/schema-utils/dist/validate.js:50:11)
  at Object.loader (/home/hussaghu/Practice/vuetest/node_modules/sass-loader/dist/index.js:36:28)

[haoqunjiang]

Well, seems vuetify plugin hasn't support sass-loader 8 yet. Then downgrade the dependency in your package.json to 7.1.0.

[ghulamhussain]

@sodatea thanks the problem is in vuetify plugin that doesn't support sass loader.