[epic] reduce repository maintenance cost

[octonato]

Right now we have too many branches and procedures to maintain Lagom.
In the coming days, I will be performing the following changes:

• Start to migrate out of TravisCI. The goal is to move to GH Actions.
• Branch 1.6.x will become main and configured as the main branch of this repo.
• Change GH Actions to build main instead of master. This include PR builds and cron jobs.
• Update dependencies. Expecting scala steward to send PRs to bring 1.6.x up-to-date. If not, we need to do manually.
• Add release by tagging from GitHub.

The reason why I'm adding the migration to GH Action as first in the list is because I believe that as soon as we change the repo to use main, Scala Steward will start to send lots of PRs. Doing that while still using TravisCI will be a waste of our time. Builds will pile-up, will require us to keep cancelling them to let other builds to pass, etc. We will be fighting an useless fight.

After that, Lagom will follow the Akka approach, ie: maintenance releases are cut from main branch.

The current master will become obsolete. Open PRs targeting master will need to be re-pointed to the new main.

We have a few PRs in master that we need to port to the new main. So far, I have identified two PRs that could be ported. None of them are crucial, so we are not in a hurry to port them.

• sbt 1.4 friendly version of Lombok #3164
• Update scalasteward settings #3238

[ihostage]

Should we move Lagom modules to this repo from Play gRPC? 🤔

[ihostage]

What a target branch will be for PR's that can't be a part of 1.6.x releases?