Version-4 dependencies update

[Ken Newman]

PostCSS dependency is outdated on the version-4 branch, and npm audit complains. (It was updated on the master/v6)

• Operating System: MacOS
• Node Version: v16.3.0
• NPM Version: 7.17.0
• webpack Version: ^4.46.0
• postcss-loader Version: ^4.3.0

Expected Behavior

npm audit wouldn't complain

Actual Behavior

npm audit complains

How Do We Reproduce?

npm audit

Just need to update the postcss dependency on the version-4 branch and publish.

[Alexander Akait]

Please update your deps locally, we use ^ for our dependencies, so you can update your deps without any problems