Removed allprojects block from root build script

[dkrasnoff]

Gradle docs says that we should avoid allprojects and subprojects blocks. In this PR I made next steps:

• Moved common logic from allprojects block in the root build script to common-conventions.gradle.kts
• Moved multiplatform connected logic from allprojects block in the root build script to multiplatform-conventions.gradle.kts
• Rewrote to kotlin dsl build script in atomicfu-gradle-plugin

Depends on

• https://github.com/Kotlin/kotlinx-atomicfu/pull/435
• https://github.com/Kotlin/kotlinx-atomicfu/pull/434

[dkrasnoff]

Depends on https://github.com/Kotlin/kotlinx-atomicfu/pull/435 and https://github.com/Kotlin/kotlinx-atomicfu/pull/434