(I'll backport)

@sadekmunawar do you think doing a release with your changes is a good idea or are you in the middle of testing mixed clusters and feel like you might find more issues?

@sadekmunawar do you think doing a release with your changes is a good idea or are you in the middle of testing mixed clusters and feel like you might find more issues?

@pjfanning I had to make one more change for the migration. I'll create a PR by the end of the week. Once that's merged, I think a release can be done.

@sadekmunawar any news on progress? If you don't have time to create a PR for the remaining issue, could you give us some pointers so someone else could create the PR instead?

@pjfanning There are two missing pieces left:

1. Deserialization of Akka cluster messages. I created the PR for this one Enable deserialization of old Akka cluster messages (mixed pekko/akka cluster) #1578
2. Supporting Akka config for JoinConfigCompatChecker. An exception is thrown here, when a Pekko node tries to find the downing path in the Akka config. I solved it by simply checking if the path exists in the config before doing the comparison.

 val downingProviderResult = {
 if (toCheck.hasPath(DowningProviderPath) && actualConfig.hasPath(DowningProviderPath))
 JoinConfigCompatChecker.checkEquality(List(DowningProviderPath), toCheck, actualConfig)
 else Invalid(im.Seq("Downing path not found. Set configuration-compatibility-check.enforce-on-join to off to enable cluster joining."))
 }

While this all that's necessary for the rolling upgrade support, I tried to find a better approach for the second problem. See this commit. I haven't had the chance to integration test this approach, yet. It should fix the exception issue, but it likely sends configs with Pekko paths to Akka nodes here

I am going to revert this due to #1586

A build of #1587 with the nightly builds temporarily enabled shows that this PR is the cause.

We can try to fix this issue again but we need a working build first - #1590.