fix: Use latest homebrew syntax for macos "depends on"

[builtbyleo]

Hey!

Kept getting this error:

Warning: Calling string comparison format for `depends_on macos:` is deprecated! Use `depends_on macos: :sequoia` instead.
Please report this issue to the productdevbook/homebrew-tap tap (not Homebrew/* repositories), or even better, submit a PR to fix it:
  /opt/homebrew/Library/Taps/productdevbook/homebrew-tap/Casks/portkiller.rb:11

I think latest version of brew deprecated this string check (https://brew.sh/2026/06/11/homebrew-6.0.0/)

[productdevbook]

Thanks for catching this! 🙏

You're right — Homebrew 6.0.0 deprecated the string comparison form for depends_on macos:. The bare symbol :sequoia keeps the same minimum (Sequoia or newer) requirement without the deprecation warning, so this is the correct fix for the generator.

I've also already applied the same change directly to the published cask in productdevbook/homebrew-tap so existing users stop seeing the warning immediately, without waiting for the next release.

Merging this so the release workflow generates the correct syntax going forward. 🚀