Update git flow support to work with git-flow-next

Lazygit has built in support for `git-flow` but it seems to only work with the legacy tooling and not `git-glow-next`. The original `git-flow` tooling has been deprecated in favour of `git-flow-next`

When I install `git-flow-next` and init the repo Lazygit errors out when I use "Show git flow options" command. This is because the `git config` format is different between the versions. I don't consider this a bug though. Instead a lack of feature.

I'd like Lazygit to work with the more recent tooling.

I have a PR ready to go with the change to make this happen but the CONTRIBUTING guidelines says I should start a discussion first.