fix(release): drop universal_binaries so homebrew_casks can find macOS archives

The v3.1.2 release failed because homebrew_casks filters archives by
goarch=[amd64 arm64], but universal_binaries.replace=true collapses the
darwin amd64+arm64 builds into a single darwin/all archive — which the
cask filter rejects.

Drop the universal_binaries block. The unpoller-mac build still produces
darwin amd64 and darwin arm64 separately, and the cask resolves them
into per-arch on_intel/on_arm blocks. Net effect on release artifacts:
the single universal macOS archive is replaced by two per-arch archives.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.goreleaser.yaml

@@ -384,11 +384,6 @@ signs:
       - unpoller-windows
       - unpoller-mac
 
-universal_binaries:
-  - replace: true
-    ids:
-      - unpoller-mac
-
 homebrew_casks:
   - name: unpoller
     ids:

CONTRIBUTING.md

@@ -19,7 +19,6 @@ As long as this document is kept up to date, this is what github does:
 -      Linux, Mac and Windows Binaries
 -      Provides a packaged source copy
 -      Builds Debian, RedHat packages
--      Builds Mac universal binary
 -      Builds Windows executable
 -      Builds numerous platform docker images and uploads them