Use general stylelint task in grunt command

[kabalin]

This change makes use of general stylelint task in grunt command, thus let grunt task to decide whether css or scss linter needs to be run (or both).

Also cleans up deprecated stylelint:less task (removed in 3.7 at MDL-64506) in a separate commit.

Fixes #314

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.25%. Comparing base (f722b93) to head (f09b894).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #320   +/-   ##
=========================================
  Coverage     88.25%   88.25%           
- Complexity      737      738    +1     
=========================================
  Files            76       76           
  Lines          2274     2274           
=========================================
  Hits           2007     2007           
  Misses          267      267           

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[junpataleta]

Thanks, @kabalin! The changes make sense and look spot-on.