Clean up build-logic

[Whathecode]

With the introduction of kotlinx-interval-datetime as a submodule, build-logic is used to apply a conventions plugin.

But, this can be cleaned up a bit:

• Central definition of versions. It is spread out between buildSrc/build.gradle.kts and build.gradle.kts at the moment, since the Nexus Gradle Plugin has to be applied to the main project.
• Double loading of publish.properties in buildSrc/build.gradle.kts and the interval.library-conventions.gradle.kts plugin.
• Add a test dependency on kotlinx.interval.testcases. But, this will require kotlinx.interval.testcases not to apply the convention plugin itself, perhaps only a subset defined in a new separate convention plugin.