Tidy up our build setup

[qwwdfsad]

Follow up of https://youtrack.jetbrains.com/issue/QA-1118/

• [ ] Update to Gradle 8
• [ ] Migrate from Groovy to Kotlin DSL everywhere
• [ ] Tidy up Kover config and make the quality bar proper
• [ ] Get rid of kludges and workarounds for old KMP, namely Idea.isActive and dynamic sourcesets hackery
• [x] Migrate to newer features of KMP, namely default source sets hierarchy
• [ ] https://youtrack.jetbrains.com/issue/KT-65886/
• [ ] https://youtrack.jetbrains.com/issue/KT-65884/

Might be helpful: two-phased cleanup and Gradle update https://github.com/Kotlin/kotlinx.coroutines/pull/3966 & https://github.com/Kotlin/kotlinx.coroutines/pull/4011

[sandwwraith]

kotlinx.serialization already uses default source sets hierarchy. Regarding idea.isActive — it is necessary in order not to import a source set with module-info.java, otherwise every common code will be red with an error about unnamed module. I am currently not aware about workarounds for that.