Refactoring build scripts

[dkrasnoff]

• [x] move plugin and dependency repositories to settings file
• [ ] change files from gradle to gradle convention plugins 1 by 1
• [x] remove usages of allprojects block
• [ ] remove usages of subprojects block
• [ ] build.gradle -> build.gradle.kts
• [ ] update targets
• [ ] remove deprecated things like buildDir
• [ ] reduce 3 ways of overriding Kotlin version (see https://github.com/Kotlin/kotlinx-atomicfu/pull/418#discussion_r1572163909)
• [ ] de-duplicate the version override in the actual project and in buildSrc (see https://github.com/Kotlin/kotlinx-atomicfu/pull/418#discussion_r1572163909)
• [ ] remove duplicated properties(for kotlin version) from TC configurations
• [ ] update integration tests to use JVM Test Suites
• [ ] move to Gradle 8.7
• [ ] enable Configuration Cache
• [ ] enable Build Cache