chore: update cssnano to 7.0

[ludofischer]

The 7.0 release is identical to the latest 6.0 release, except it requires Node.js 18 as a minimum

This PR contains a:

• [ ] bugfix
• [ ] new feature
• [x] code refactor
• [ ] test update
• [ ] typo fix
• [ ] metadata update

Motivation / Use-Case

I don't plan to make new releases to cssnano 6.x, so to receive updates, css-minimizer-webpack-plugin should switch to 7.0

Breaking Changes

There are no breaking changes because css-minimizer-webpack-plugin already requires node >= 18.12

Additional Info

https://github.com/cssnano/cssnano/releases

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.09%. Comparing base (2ce7dc0) to head (6d6078f). Report is 5 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #263   +/-   ##
=======================================
  Coverage   96.09%   96.09%           
=======================================
  Files           3        3           
  Lines         282      282           
  Branches      111      110    -1     
=======================================
  Hits          271      271           
  Misses         11       11           

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