Fix deprecated depends_on macos: syntax in generated formula

Homebrew now deprecates having a bare `depends_on :macos` (auto-emitted
by GoReleaser for darwin-only targets) alongside a top-level versioned
`depends_on :macos => :sequoia`. Wrap the version constraint in an
`on_macos` block, which is the form Homebrew's deprecation message
recommends, and switch to the blessed `depends_on macos: :sequoia`
keyword syntax (the `=> :sequoia` / string ">= :sequoia" forms are both
deprecated). Semantics are unchanged: macOS Sequoia or newer.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.goreleaser.yml

@@ -30,4 +30,6 @@ brews:
     description: Software networking with isolation for Tart
     skip_upload: auto
     custom_block: |
-      depends_on :macos => :sequoia
+      on_macos do
+        depends_on macos: :sequoia
+      end