Bump Kotlin to 1.5.21

[cortinico]

This is a follow up to #3921 and #3941

[codecov]

Codecov Report

Merging #3956 (10cfa98) into main (b2f277c) will decrease coverage by 0.13%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##               main    #3956      +/-   ##
============================================
- Coverage     83.58%   83.44%   -0.14%     
- Complexity     3123     3149      +26     
============================================
  Files           456      456              
  Lines          9014     9014              
  Branches       1752     1754       +2     
============================================
- Hits           7534     7522      -12     
- Misses          564      565       +1     
- Partials        916      927      +11     

Impacted Files	Coverage Δ
...ab/arturbosch/detekt/rules/bugs/MissingWhenCase.kt	88.00% <ø> (ø)
...sch/detekt/rules/coroutines/SleepInsteadOfDelay.kt	72.00% <0.00%> (-8.00%)	⬇️
...arturbosch/detekt/generator/collection/KDocTags.kt	62.50% <0.00%> (-5.00%)	⬇️
...h/detekt/rules/style/DataClassContainsFunctions.kt	95.00% <0.00%> (-5.00%)	⬇️
...rturbosch/detekt/rules/bugs/UnnecessarySafeCall.kt	73.91% <0.00%> (-4.35%)	⬇️
...io/github/detekt/report/sarif/SarifOutputReport.kt	90.90% <0.00%> (-3.04%)	⬇️
...rturbosch/detekt/rules/style/UnusedPrivateClass.kt	70.66% <0.00%> (-2.67%)	⬇️
...ekt/generator/collection/DocumentationCollector.kt	85.00% <0.00%> (-2.50%)	⬇️
...t/rules/style/UtilityClassWithPublicConstructor.kt	93.61% <0.00%> (-2.13%)	⬇️
...ain/kotlin/io/gitlab/arturbosch/detekt/cli/Spec.kt	88.23% <0.00%> (-1.97%)	⬇️

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