formula: update some DSL style in comments

[cho-m]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

---

So that documentation is updated at places like https://docs.brew.sh/rubydoc/Formula.html#depends_on-class_method

[Copilot]

Pull Request Overview

This PR updates code examples in the formula documentation to reflect current best practices and syntax conventions.

• Replaces deprecated conditional syntax with modern on_<version> block syntax
• Updates fails_with example to use keyword arguments instead of hash rockets
• Standardizes string quotes to use double quotes

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[MikeMcQuaid]

Thanks, looks good!