Move to "JetBrains" GitHub organization

[ndkoval]

As Lincheck is not a Kotlin-specific framework, the repository has been recently moved from "Kotlin" GitHub organization to "JetBrains." However, to complete the transition, two additional steps are necessary:

1. We need to change the package of Lincheck classes from org.jetbrains.kotlinx.lincheck to org.jetbrains.lincheck. Yes, this is a backward incompatible change, but it reduces the association with Kotlin.
2. Next, we need to change the artifact location from org.jetbrains.kotlinx:lincheck:<version> to org.jetbrains.lincheck:lincheck:<version>.