Upgrade to Gradle 8.1.1

[tristancaron]

30 suspects that Gradle may be the culprit for poor performances.

Current version of Gradle (6.x) was released in 2019. Performance improvements can be expected with this year version (8.x)

Few breacking changes forced us to modify our configuration:

• mainClassName is not supported anymore
• shadowJar needed to be updated because old version relied on some removed Gradle API
• jcenter is deprecated

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[ErikSchierboom]

@tristancaron I'm happy to accept this, but this change only influences the build performance as we've switched to Maven for running the tests: https://github.com/tristancaron/kotlin-test-runner/blob/ccc45447fa7e29b4582df71cabe98cb3ea511918/src/main/kotlin/exercism/kotlin/autotests/executor/executor.kt#L56

[tristancaron]

@tristancaron I'm happy to accept this, but this change only influences the build performance as we've switched to Maven for running the tests: https://github.com/tristancaron/kotlin-test-runner/blob/ccc45447fa7e29b4582df71cabe98cb3ea511918/src/main/kotlin/exercism/kotlin/autotests/executor/executor.kt#L56

You are correct. Thank you for pointing it out.