Use Gradle Toolchain to pin JDK version

[Okeanos]

While poking around the project I noticed that the build.gradle.kts declares the the Java compatibility but doesn't enforce a consistent JVM environment.

Gradle offers Toolchains to ensure that projects can be build correctly with the expected JVM compile environment.

In order to improve reproducibility of the builds and make build requirements clearer, Gradle allows configuring toolchains on both project and task levels.

Unless there are specific reasons that I fail to see I would suggest adding such a declaration to make working with the project source code a little more robust.

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Addendum: I would also suggest switching away from sourceCompatibility/targetCompatibility to release instead.