Roll new changes into `package:mdc_web` then update in samples_index

[parlough]

We currently have to override the sass dependency due to a previous issue with Material Components Web which the fix for has not yet rolled into package:mdc_web. Due to changes in modern sass versions, we also have to restrict sass_builder to an old version.

We should incorporate the new changes into package:mdc_web, then update the packages here and remove the necessary overrides.

[domesticmouse]

The last publish date of mdc_web is 2 years ago, according to https://pub.dev/packages/mdc_web

@johnpryan are you in a place to do this work?