Update dependency com.diffplug.spotless:spotless-plugin-gradle to v6.21.0

[renovate]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
com.diffplug.spotless:spotless-plugin-gradle	6.20.0 -> 6.21.0

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Configuration

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[codecov]

Codecov Report

Merging #759 (592a44d) into main (f3a0a7a) will not change coverage.
Report is 1 commits behind head on main.
The diff coverage is n/a.

❗ Current head 592a44d differs from pull request most recent head b3d52f7. Consider uploading reports for the commit b3d52f7 to get more accurate results

@@            Coverage Diff            @@
##               main     #759   +/-   ##
=========================================
  Coverage     82.18%   82.18%           
  Complexity      179      179           
=========================================
  Files            49       49           
  Lines          1443     1443           
  Branches        189      189           
=========================================
  Hits           1186     1186           
  Misses          165      165           
  Partials         92       92           

Flag	Coverage Δ
unittests	82.18% <ø> (ø)

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