ci: use GitHub native release notes in goreleaser

Switches changelog generation to github-native so GoReleaser uses
GitHub's release notes API, which respects .github/release.yml and
produces categorized, PR-based release notes.

Author: justinharringa

.goreleaser.yml

@@ -1,5 +1,7 @@
 version: 2
 project_name: ci-result-to-slack
+changelog:
+  use: github-native
 builds:
   - env: [CGO_ENABLED=0]
     main: ./cmd/ci-result-to-slack