Make `unused-ignore-comment` enabled by default

[AlexWaygood]

We made this rule disabled by default in https://github.com/astral-sh/ruff/pull/17955, on the following grounds:

Right now it seems like there are too many situations where:

1. There is a typing error on a line that all other type checkers detect
2. The user has suppressed that error for Reasons with a type: ignore comment
3. But we don't yet detect the typing error due to a missing feature, leading us to complain about the unused ignore comment

The rule feels like more of a lint anyway, and mypy's version is opt-in. I'd like for it to be enabled by default for the GA release, but right now it feels like we're getting a lot of user questions about it. Let's make it disabled-by-default until we're closer to feature-parity with other type checkers.

I think we all agree that we'd like it to be enabled by default for the GA release. This issue exists so we don't forget to do so.