Project: use get_original_latest_version in form (#12460)
This is the same logic we have in
https://github.com/readthedocs/readthedocs.org/blob/a8a29fe29292027a4e2a44cc468cea4adc70e390/readthedocs/projects/models.py#L1164-L1181
and it already excludes latest
I'm doing small refactors to rely less on get_default_branch, as that
contains some extra logic that we already run to have latest always in
sync.