GroupPermissionsHashGeneratorInterface received a backwards incompatible change

For the full description of what has changed and why this was done, see the change record here: https://www.drupal.org/node/3035038. This CR aims to describe exactly what the backwards incompatible change was.

We used to generate hashes per audience by employing the generateAnonymousHash() and generateAuthenticatedHash() methods. With the new permission system, this no longer made sense and the method was replaced by a more generic generateHash() method.

There are two main reasons behind this BC break:

1. Because the change is security related, we could not leave methods with names that imply something they're no longer doing, i.e.: We thought about making the old methods a wrapper of the new one but that might have caused dangerous confusion.
2. The hash generator is a rather internal class and might/should actually be indicated as such using an @internal doxygen flag. This might yet happen before a full release.