Kotlin 2.1.0 will drop support for language version 1.4 and 1.5. But we need need to use older api/language version so that DGP will be compatible with Gradle 7.6.
To be able to migrate DGP (and dependent modules) to it, we will need to use fixed compiler version for compiling which still supports old language versions (e.g 2.0.20).
Similar changes were already implemented in kotlinx-benchmark.