Support 3.14 annotations in converter functions

While #1451 added support for forward references in the classes themselves, forward references in converter functions still fail due to the use of inspect.signature which by default uses Format.VALUE when getting annotations.

Example:

from attrs import define, field

@define
class Works:
    x: unknown1

def converter(x: unknown2 | str) -> str:
    return str(x)

@define
class Fails:
    x: str = field(converter=converter)

Result:

NameError: name 'unknown2' is not defined

Cause:
https://github.com/python-attrs/attrs/blob/1315e42fe51743fe1c3529da6792dcda2ef6cc1f/src/attr/_compat.py#L50

For Python 3.14 annotations you now need:

self.sig = inspect.signature(callable, annotation_format=annotationlib.Format.FORWARDREF)

Start in src/attr/_compat.py at the inspect.signature call linked in the issue, then reproduce the Fails example with the converter annotation. Confirm that the converter accepts unresolved forward references under Python 3.14 without raising NameError; the issue does not name a specific test file.