Setup repo

- [x] Rename to main branch
- [x] Discord webhook
- [ ] Rename to play-flyway?
- [ ] Change groupId to `org.playframework[.flyway]`?
- [ ] https://github.com/VirtusLab/scala-steward-repos/blob/main/repos-github.md (depends on previous todos)
- [ ] Update README
- [ ] GitHub actions Workflows
- [ ] Branch protection(s)
- [ ] License