packaging: migrate Homebrew publishing from brews to homebrew_casks

[cybertec-adrian]

goreleaser deprecated the brews: (formula) pipe in v2.16 in favour of homebrew_casks: — goreleaser check fails the formula config as "uses deprecated properties". Migrate the Homebrew publish accordingly:

• brews: -> homebrew_casks:
• install block (bin.install) -> binaries: [pg_hardstorage]
• drop the formula-only test: block
• add a postflight hook that strips the Gatekeeper quarantine xattr on macOS (OS.mac?-guarded): the release binaries are cosign-signed but not Apple-notarised, so without it macOS refuses to run the binary ("pg_hardstorage is damaged and cannot be opened")

The end-user install command is unchanged
(brew install cybertec-postgresql/tap/pg_hardstorage), still covering macOS arm64 and Linux amd64/arm64, still no hard PostgreSQL dependency.

Verified with goreleaser check (validated, no deprecation) and a full goreleaser release --snapshot that generated a correct Casks/pg_hardstorage.rb (per-platform url/sha256, binary, postflight, caveats). Since no tag release has populated the tap yet, no Formula cleanup / tap_migrations.json is needed.

Summary

Migrate Homebrew publishing from the deprecated brews: (formula) pipe to homebrew_casks:. goreleaser deprecated brews: in v2.16, so the previous config would fail goreleaser check. The end-user install command is unchanged (brew install cybertec-postgresql/tap/pg_hardstorage); the macOS path now strips the Gatekeeper quarantine xattr since the binaries are cosign-signed but not Apple-notarised.

Type

• Bug fix
• New feature
• Refactor (no behaviour change)
• Documentation
• Test infrastructure
• Packaging / release

Tests

• make check passes locally (vet + race tests)
• New tests added for the changed behaviour, or there's a clear reason none exist
• Integration tests pass (make test-integration) where touched

Compatibility

• No on-disk manifest schema changes (or: schema bumped + 24-month back-read preserved)
• No CLI / API contract changes (or: documented + bumped)
• No new external dependencies (or: justified in the description)

Checklist

• Maintainer-attribution authoring (Author: Hans-Jürgen Schönig <hs@cybertec.at>)
• No AI-attribution lines anywhere
• CHANGELOG.md updated under the unreleased section
• Comments explain WHY (not WHAT) where the code isn't self-evident

Reviewer / release notes:

• Supersedes the brews: config from packaging: publish a Homebrew formula on release #5 (not yet exercised by a release,
  so the tap is still empty — no Formula/ cleanup or tap_migrations.json
  needed).
• First real population of the tap happens on the next v* tag (e.g.
  v1.0.1). After that, test: brew install cybertec-postgresql/tap/pg_hardstorage
• The postflight xattr strip is a Gatekeeper workaround for un-notarised
  binaries; revisit if/when Apple notarisation is added to the release.