Add junit-platform-launcher to test runtime

[kahgoh]

This is fix the following warning about using deprecated Gradle features:

The automatic loading of test framework implementation dependencies
has been deprecated. This is scheduled to be removed in Gradle 9.0.
Declare the desired test framework directly on the test suite or
explicitly declare the test framework implementation dependencies on
the test's runtime classpath. Consult the upgrading guide for
further information:
https://docs.gradle.org/8.7/userguide/upgrading_version_8.html#test_framework_implementation_dependencies

pull request

The deprecation warning was mentioned in the forums https://forum.exercism.org/t/deprecated-gradle-features-warning-in-the-java-track/14906

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[github-actions]

This PR touches files which potentially affect the outcome of the tests of an exercise. This will cause all students' solutions to affected exercises to be re-tested.

If this PR does not affect the result of the test (or, for example, adds an edge case that is not worth rerunning all tests for), please add the following to the merge-commit message which will stops student's tests from re-running. Please copy-paste to avoid typos.

[no important files changed]

For more information, refer to the documentation. If you are unsure whether to add the message or not, please ping @exercism/maintainers-admin in a comment. Thank you!

[kahgoh]

When merging, I think we can add the [no important files changed] because this doesn't really change the nature of the tests.

[kahgoh]

@manumafe98, could you please review this?