Refactoring build scripts

[x] move plugin and dependency repositories to settings file
[x] change files from gradle to gradle convention plugins 1 by 1
[x] remove usages of allprojects block
[x] remove usages of subprojects block
[x] build.gradle -> build.gradle.kts
[x] update targets
[x] remove deprecated things like buildDir
[x] reduce 3 ways of overriding Kotlin version (see https://github.com/Kotlin/kotlinx-atomicfu/pull/418#discussion_r1572163909)
[x] de-duplicate the version override in the actual project and in buildSrc (see https://github.com/Kotlin/kotlinx-atomicfu/pull/418#discussion_r1572163909)
[x] move to Gradle 8.7
[x] enable Configuration Cache
[x] enable Build Cache
[x] remote Build Cache (CI)
[x] Build scans
[x] Use cache-redirector

Kotlin / kotlinx-atomicfu