Create dedicated repository for sustaining/15.x branch

[pavelhoral]

Quite a lot of tools we are using (DependaBot, SonarQube, CodeQL) are working with repository's default branch only. With migration to Jakarta, we still want to support sustaining/15.x branch and have all the benefits / assurances those tools bring.

The only solution I can think of is to simply create a fork of Wren:AM as wrenam-15.x repository, where the main branch will be sustaining/15.x and we can add GitHub workflow that will periodically update the main branch (or maybe GitHub can do that automatically for us). Then we can setup all the tools on that repository similar to how we have them set up here.

[pavelhoral]

Btw. I am not sure if CodeQL / DependaBot is really not able to watch non-default branch. If they actually are able to watch other branches, then this issue can be closed. We don't need SonarQube Cloud to track code / security metrics in time.