Finish Kotlin migration

[Goooler]

Closes #980.

---

• [x] CHANGELOG's "Unreleased" section has been updated, if applicable.

[Goooler]

~/IdeaProjects/shadow/src/main/kotlin git:[main]
cloc .
      45 text files.
      45 unique files.                              
       0 files ignored.

github.com/AlDanial/cloc v 2.02  T=0.02 s (2730.9 files/s, 206092.2 lines/s)
-------------------------------------------------------------------------------
Language                     files          blank        comment           code
-------------------------------------------------------------------------------
Kotlin                          45            442            348           2606
-------------------------------------------------------------------------------
SUM:                            45            442            348           2606
-------------------------------------------------------------------------------

[Goooler]

• https://scans.gradle.com/s/zdxkctgrn36vw/tests/overview
• https://scans.gradle.com/s/n3hu54ok26dg6/tests/overview