attr.asdict -> attrs.asdict regression for sets of instances

Hard coding retain_collection_types=True in attrs.asdict means that you can no longer convert objects that contain sets of other objects. Reproducer:

#!/usr/bin/env python
# -*- coding: utf-8 -*-

import attr
import attrs
from attrs import frozen

@frozen
class Foo:
    x: int

@frozen
class Bar:
    foos: set[Foo]

# works!
print(attr.asdict(Bar({Foo(3)})))

# fails with:
# Traceback (most recent call last):
#   File "/tmp/regression.py", line 16, in <module>
#     print(attrs.asdict(Bar({Foo(3)})))
#           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
#   File "/home/x/.local/lib/python3.11/site-packages/attr/_next_gen.py", line 214, in asdict
#     return _asdict(
#            ^^^^^^^^
#   File "/home/x/.local/lib/python3.11/site-packages/attr/_funcs.py", line 75, in asdict
#     rv[a.name] = cf(
#                  ^^^
# TypeError: unhashable type: 'dict'
print(attrs.asdict(Bar({Foo(3)})))

Run the supplied Python reproducer first to confirm the TypeError. Then read attr/_next_gen.py and attr/_funcs.py, which appear in the traceback, and trace how attrs.asdict handles the set of Foo instances. Done means attrs.asdict(Bar({Foo(3)})) converts successfully without the unhashable-dict failure.