Homogeneize the base branch argument

[juliojgd]

The argument used to specify the base branch is in gh sherpa create-branch and gh sherpa create-pr is :
-b, --base string base branch for checkout. Use the default branch of the repository if it is not set
Both for create-branch and create-pr commands.

But in gh cli the argument with the same meaning is (gh pr create):

-B, --base branch The branch into which you want your code merged

I think that, to keep the coherence in the gh tooling ecosystem and decrease the cognitive load for developers using both tools, sherpa should adopt the uppercase -B argument, deprecating the lowercase -b one.
So the argument in sherpa should be:

-B, --base branch         base branch for checkout. Use the default branch of the repository if it is not set
-b                        (deprecated) Same  as -B, --base

This is a minor improvement that keeps compatibility but improves regularity among tools.