Release role: use feature freeze for all betas except b1, and all release candidates; allow to skip prepare step

[felixfontein]

Ref: ansible-community/ansible-build-data#391 (comment)

[felixfontein]

Ok, I changed the behavior of the role to no longer automatically skip the prepare step if the .deps file is missing (after all, the prepare step does a lot more than creating the .deps file, and this behavior isn't compatible with #599 either).

So now to skip running prepare when the .deps file is present, you need to explicitly set antsibull_skip_prepare=true. I've marked this as a breaking change in the changelog.

[felixfontein]

Thanks for reviewing this @gotmax23!