Update Gradle Wrapper to 8.3

[beatngu13]

Proposed commit message:

Update Gradle Wrapper to 8.3 (#760)

See: https://github.com/gradle/gradle/releases/tag/v8.3.0

This also requires an update of the Checkstyle plugin to 10.12.3.
The `checkstyleMain` and `checkstyleTest` tasks otherwise fail with:

```text
* What went wrong:
Execution failed for task ':checkstyleTest'.
> A failure occurred while executing org.gradle.api.plugins.quality.internal.CheckstyleAction
   > input file /Users/kruse/dev/workspaces/private/junit-pioneer/build/reports/checkstyle/main.xml does not exist
```

(Respectively, `test.xml`.)

PR: #760

---

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