dafny: align deprecation with dotnet@6 and drop Ventura bottle

[cho-m]

• Have you followed the guidelines for contributing?
• Have you ensured that your commits follow the commit style guide?
• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with HOMEBREW_NO_INSTALL_FROM_API=1 brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
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Ventura has some issue with dotnet@6 so needed to drop the bottle. Do the same here to avoid failing tests.

Should be able to trigger a bottle request for Sonoma now.

Given we previously weren't able to bottle on dotnet@6 on Sonoma, the issue is most likely Xcode 15.0 - 15.2 related and fixed in Xcode 15.3. Could be something like the new linker.

[chenrui333]

• Support dotnet 8 dafny-lang/dafny#4948
• Update to .NET 8 dafny-lang/dafny#5322