Switch to Apache Pekko from Akka

[hubertp]

Akka 2.6.x has reached End Of Life and switched to a non-open source license. This means that we are not receiving any support or security updates since about 02/2024. This was known.

While there is a plan in progress to integrate polyglot Ydoc with Insights, an alternative solution exists if the former is hunted by delays (and since it is very much experimental... there will be delays). An open-source fork of Akka exists which receives security updates and patches. That could prove a useful solution and potentially an easy switch.