Remove internal @deprecated decorator #3301

The internal plasmapy.utils.decorators.deprecation.deprecated decorator was deprecated in #3300 in favor of warnings.deprecated, which was added in Python 3.13 as a way to standardize deprecation.

Once PlasmaPy drops Python 3.12 support around October 2026, we should remove the internal @deprecated decorator and adopt @warnings.deprecated instead.

See also #3075.