Refactor use of __framework_name__ in framework classes

[metrizable]

Is your feature request related to a problem? Please describe.

The framework classes define a __framework_name__ attribute used in multiple places that may lead to breakage without warning. From the docs:

Any use of __*__ names, in any context, that does not follow explicitly documented use, is subject to breakage without warning.

Describe the solution you'd like

Protected attributes should use, by convention, a single leading underscore, like

class Framework(EstimatorBase):

    _framework_name = None

Describe alternatives you've considered

• One could forego the leading single underscore, but it lacks the convention of letting code authors know that the attribute is intended to be protected.
• One could use a leading dunder, but the class-private name may have unintended consequences regarding expectation of the existing use in inheritance and name mangling.

Additional context

n/a