OneDev migration: fix broken migration caused by various REST API changes in OneDev 7.8.0 and later (#35216)

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)
- set milestone closed state if milestone is closed in OneDev
- moved memory allocation for milestone JSON decoding into for loop
(which gets 100 milestones per iteration) to fix wrong due dates when
having more than 100 milestones

Author: DevCharly