Stop supporting "old" K/N memory model in the favour of new one

[qwwdfsad]

To significantly simplify maintenance and implementation of new features for kotlinx.coroutines 1.7.0.

The new memory model will be enabled by default in Kotlin 1.7.20, and the old one will likely be deprecated in 1.8.0. We expect to release kotlinx.coroutines 1.7.0 with Kotlin 1.8.0, so at this moment it will be no longer relevant