Update to postcss 8.x.

• autoprefixer 10.x
• find-unused-sass-variables - https://github.com/XhmikosR/find-unused-sass-variables/pull/95 - needs a new npm version, but I'd like to update commander too https://github.com/XhmikosR/find-unused-sass-variables/pull/89#issuecomment-696614770 Released v3.0.0 without the commander update for now
• postcss-cli 8.x
• stylelint - they are working on it upstream

Current dependency tree:

C:\Users\xmr\Desktop\bootstrap>npm ls postcss
bootstrap@5.0.0-alpha2 C:\Users\xmr\Desktop\bootstrap
+-- find-unused-sass-variables@3.0.0
| +-- postcss@8.1.4  deduped
| `-- postcss-scss@3.0.2
|   `-- postcss@8.1.4  deduped
+-- postcss@8.1.4
+-- stylelint@13.7.2
| +-- autoprefixer@9.8.6
| | `-- postcss@7.0.35  deduped
| +-- postcss@7.0.35
| +-- postcss-less@3.1.4
| | `-- postcss@7.0.35
| +-- postcss-safe-parser@4.0.2
| | `-- postcss@7.0.35
| +-- postcss-sass@0.4.4
| | `-- postcss@7.0.35
| +-- postcss-scss@2.1.1
| | `-- postcss@7.0.35
| `-- sugarss@2.0.0
|   `-- postcss@7.0.35
`-- stylelint-config-twbs-bootstrap@2.1.0
  `-- stylelint-order@4.1.0
    +-- postcss@7.0.35
    `-- postcss-sorting@5.0.1
      `-- postcss@7.0.35