Update plugin dokka to v1.6.21

[renovate]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
org.jetbrains.dokka	1.6.20 -> 1.6.21

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Name	Status	Preview	Updated
detekt	✅ Ready (Inspect)	Visit Preview	Apr 25, 2022 at 7:23PM (UTC)

[codecov]

Codecov Report

Merging #4770 (a7e2abc) into main (f5ceac7) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               main    #4770   +/-   ##
=========================================
  Coverage     84.69%   84.69%           
  Complexity     3413     3413           
=========================================
  Files           490      490           
  Lines         11232    11232           
  Branches       2065     2065           
=========================================
  Hits           9513     9513           
  Misses          675      675           
  Partials       1044     1044           

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