[BitSet] Amend return value of update(with:) method

[benrimmington]

The implementation and tests didn't match the documentation.
The tests are amended to rely on the expected Set<Int> results.

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (if appropriate).
• I've added benchmarks covering new functionality (if appropriate).
• I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• I've updated the documentation if necessary.

[lorentey]

Good catch! What a thinko

[lorentey]

Hmph; workflow is not triggering. I'll close and reopen this PR to try to get things in motion -- apologies for the noise.

[lorentey]

@aluco100 Please stop reviewing PRs in this repository; it is a nuisance, and I'll be forced to take action if it continues.

[aluco100]

@aluco100 Please stop reviewing PRs in this repository; it is a nuisance, and I'll be forced to take action if it continues.

Hey @lorentey the idea to have an open source project is to contribute between each other right? Otherwise this repo should be private if you do not want any external inputs. Please I only want to contribute and if you put a pr ready to review I believe is ready to ship. Saying that I didn't encounter any issues to that so please if you believe I block you please consider this repo as private or please tell me to avoid bothering you.

[xwu]

Posting unsolicited checkmarks to PRs that you are not authoring, for which you are not code owner or a requested reviewer, is going to be regarded as nuisance behavior in essentially any repo. You are causing notifications to be sent to everybody who is involved or even just watching the repo with no content whatsoever. You should not be surprised if you get banned from repos for continuing this behavior, and it is likely your account will eventually be reported to GitHub as well.

If you are enthusiastic about a contribution you may consider pushing the button below the PR summary to react with ❤️. You are of course welcome to raise specific concerns--for example, when you notice an error in the code--by writing comments in PRs. For substantive contributions, you may refer to the following link for this repo's specific guidelines:
https://github.com/apple/swift-collections?tab=readme-ov-file#contributing-to-swift-collections