Revert "Bump Kotlin to v1.5.20"

[cortinico]

This PR reverts #3921

See my comment here: #3921 (comment)

Since this is blocking the release process, I'd suggest we:

1. Revert this PR
2. Craft a 1.18.0-RC without this PR
3. Wait for 1.5.21 to release the 1.18.0 stable

[codecov]

Codecov Report

Merging #3941 (4cab22d) into main (b2aba98) will increase coverage by 0.13%.
The diff coverage is n/a.

❗ Current head 4cab22d differs from pull request most recent head d556ace. Consider uploading reports for the commit d556ace to get more accurate results

@@             Coverage Diff              @@
##               main    #3941      +/-   ##
============================================
+ Coverage     83.44%   83.58%   +0.13%     
+ Complexity     3149     3123      -26     
============================================
  Files           456      456              
  Lines          9014     9014              
  Branches       1754     1752       -2     
============================================
+ Hits           7522     7534      +12     
+ Misses          565      564       -1     
+ Partials        927      916      -11     

Impacted Files	Coverage Δ
...ab/arturbosch/detekt/rules/bugs/MissingWhenCase.kt	88.00% <ø> (ø)
...ain/kotlin/io/gitlab/arturbosch/detekt/cli/Spec.kt	90.19% <0.00%> (+1.96%)	⬆️
...t/rules/style/UtilityClassWithPublicConstructor.kt	95.74% <0.00%> (+2.12%)	⬆️
...ekt/generator/collection/DocumentationCollector.kt	87.50% <0.00%> (+2.50%)	⬆️
...rturbosch/detekt/rules/style/UnusedPrivateClass.kt	73.33% <0.00%> (+2.66%)	⬆️
...io/github/detekt/report/sarif/SarifOutputReport.kt	93.93% <0.00%> (+3.03%)	⬆️
...rturbosch/detekt/rules/bugs/UnnecessarySafeCall.kt	78.26% <0.00%> (+4.34%)	⬆️
...arturbosch/detekt/generator/collection/KDocTags.kt	67.50% <0.00%> (+5.00%)	⬆️
...h/detekt/rules/style/DataClassContainsFunctions.kt	100.00% <0.00%> (+5.00%)	⬆️
...sch/detekt/rules/coroutines/SleepInsteadOfDelay.kt	80.00% <0.00%> (+8.00%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b2aba98...d556ace. Read the comment docs.

[schalkms]

Sounds like a plan!