Create first release branch

Related discussions:

• versioning scheme (-> branch name): https://gitlab.com/postmarketOS/postmarketos/-/issues/20
• how to handle updates from upstream? https://gitlab.com/postmarketOS/postmarketos/-/issues/21 (-> what should we branch at which version?)

What are our plans for creating the first release branch?

• Which packages should we branch? (everything except for device/testing?)
• Should we use git-filter-branch for that?
• What about new packages in Alpine that are only in edge, which we are using? For example, all of phosh?
  • Looks like we need to fork them, but the rust package squeekboard may not build again?
  • How do we keep packages forked from Alpine edge to postmarketOS latest in sync, create a job for monitoring?

For the sake of testing, we might start with a minimal branch that only has the cross compiling packages etc. But the questions above should be addressed, input welcome.