chore(postcss): deprecate config support

[manniL]

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Feature deprecation

Description

Using a postcss.config.js or a similar file will not apply any of the default plugins coming with Nuxt which could lead to severe issues (like not resolving aliases).

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly. (PR: #)
• I have added tests to cover my changes (if not applicable, please state why)
• All new and existing tests are passing.

[codecov-io]

Codecov Report

Merging #5419 into dev will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              dev    #5419      +/-   ##
==========================================
+ Coverage   96.18%   96.19%   +<.01%     
==========================================
  Files          74       74              
  Lines        2544     2549       +5     
  Branches      648      649       +1     
==========================================
+ Hits         2447     2452       +5     
  Misses         81       81              
  Partials       16       16

Impacted Files	Coverage Δ
packages/webpack/src/utils/postcss.js	93.22% <100%> (+0.62%)	⬆️

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