Update plugin spotless to v6.21.0

[renovate[bot]]

This PR contains the following updates:

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

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Configuration

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♻ Rebasing: Whenever PR becomes conflicted, or you tick the rebase/retry checkbox.

🔕 Ignore: Close this PR and you won't be reminded about this update again.

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Test Results

125 tests  ±0   125 :heavy_check_mark: ±0   26s :stopwatch: -1s   62 suites ±0       0 :zzz: ±0    62 files   ±0       0 :x: ±0 

Results for commit 52af74ef. ± Comparison against base commit d59d3372.