Let's move to Bazel Registry and Kotlin 2

Hi

As you know, https://registry.bazel.build/ is a game changer to Bazel projects and Kotlin 2 is quite a major release. I have been waiting for some time, so let's upgrade.

A few things to do (https://github.com/grpc/grpc-kotlin/pull/640):
1. Upgrade bazel version & move to bzlmod.
1. Pick the right version for Kotlin-related packages.

Cheers