update examples/brushnet/test_brushnet_sdxl.py

[yuanhangio]

What does this PR do?

This PR proposes to change the scheduler in examples/brushnet/test_brushnet_sdxl.py, which improves the image generation quality. Here is a comparison of the images before and after the change.	UniPCMultistepScheduler	DPMSolverMultistepScheduler

Before submitting

• [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• [x] Did you read the contributor guideline?
• [x] Did you read our philosophy doc (important for complex PRs)?
• [ ] Was this discussed/approved via a GitHub issue or the forum? Please add a link to it if that's the case.
• [ ] Did you make sure to update the documentation with your changes? Here are the documentation guidelines, and here are tips on formatting docstrings.
• [ ] Did you write any new necessary tests?

Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag members/contributors who may be interested in your PR. @juxuan27