Deprecate `fallback_branch` and use `fallback_ref`

[nikolas-burkoff]

Thereby allowing tags to be the fallback

Do this here:
https://github.com/openpharma/staged.dependencies/blob/main/R/dependencies.R#L74
https://github.com/openpharma/staged.dependencies/blob/main/R/dependencies_app.R#L33

and it ends up being used here from e.g. here