Extract classical transport to dedicated modules and provide both Netty 4.1 and Netty 4.2 implementations?

Some users are using an old version of Netty, eg, Netty 4.1.x. If the user upgrades to the new version of Pekko 1.2.x but does not upgrade other dependencies, then an error will happen...

We encounter this problem at $Work and have to exclude the Netty 4.2 dependency.

Should we split the Netty transport into two modules`transport-netty41` and `transport-netty42`?

refs: https://github.com/apache/pekko/pull/1794