OneDev migration: fix broken migration caused by various REST API changes in OneDev 7.8.0 and later

- in REST urls use `~api` instead of `api`
- check minimum required OneDev version before starting migration
- required OneDev version is now 12.0.1
  (older versions do not offer necessary API: https://code.onedev.io/onedev/server/~issues/2491)
- support migrating OneDev subprojects (e.g. http:/onedev.host/projectA/subProjectB)

Author: DevCharly