Mark legacy cluster arguments as deprecated

[samutamm]

Close #556

• Clarify configuration arguments.

[codecov-commenter]

Codecov Report

Merging #557 (9787d53) into main (0dc0d5a) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #557      +/-   ##
==========================================
+ Coverage   78.58%   78.61%   +0.03%     
==========================================
  Files          95       95              
  Lines       10940    10940              
==========================================
+ Hits         8597     8601       +4     
+ Misses       1860     1856       -4     
  Partials      483      483              

Impacted Files	Coverage Δ
pkg/cluster/config.go	89.25% <ø> (ø)
pkg/filter/validator/basicauth.go	80.73% <0.00%> (+1.83%)	⬆️

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