update to Gradle 8.4

[ErwinOlie]

The changes in this PR consist of:

• replaced the deprecated jcenter repository by mavenCentral
• added missing settings.gradle.kts in all projects
• ran gradle wrapper --gradle-version 8.4 --distribution-type bin, this automatically updated the wrappers, wrapper properties and wrapper scripts to the latest versions. Note that most changed lines are because of this automated update.
• removed explicit compile(kotlin("stdlib")). The latest gradle versions removed compile which should be replaced by implementation or api, but in the current version of the Gradle Kotlin plugin the stdlib is added automatically.
• updated Dockerfile to build with Gradle 8.4

This is my first PR here. Feedback is welcome :)

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[iHiD]

(@ErikSchierboom See conversation at https://forum.exercism.org/t/kotlin-maintainer/7938/4)

[ErwinOlie]

Updated the description to tell what changed. I'll be adding a benchmark soon (but I don't expect any significant changes since maven is currently used for the test runs, if i'm not mistaken).

[ErwinOlie]

I time'd the docker run command in run-tests-in-docker.sh five times in a row on my own machine for both. Is that a good approach? I would like to give the performance problems a look as well later, can I use the same approach as a benchmark for that?

Before (Gradle 8.1.1 with some 6.0.1 wrappers)
real    0m54.529s
real    0m53.275s
real    0m53.502s
real    0m47.697s
real    0m54.221s
---
average 52.645s

After (Gradle 8.4.0)
real    0m57.295s
real    0m48.819s
real    0m57.015s
real    0m49.466s
real    0m49.706s
---
average 52.460s

I would argue that the little improvement can be seen as insignificant, especially since no change is expected.