ci: bump golangci-lint-action to v9 for Node 24

Clears the remaining Node.js 20 deprecation warning. v9 is the release
that migrates the action runtime from node20 to node24 and remains
compatible with the pinned golangci-lint v2.1.6 (requires >= v2.1.0).

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

Author: erikhoward

.github/workflows/ci.yml

@@ -90,7 +90,7 @@ jobs:
           cache: true
 
       - name: golangci-lint
-        uses: golangci/golangci-lint-action@v7
+        uses: golangci/golangci-lint-action@v9
         with:
           version: v2.1.6