fix(ci): upgrade actions/cache from v2 to v4

actions/cache v2 has been deprecated and causes CI failures.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -31,12 +31,12 @@ jobs:
         echo "::set-output name=go-build::$(go env GOCACHE)"
         echo "::set-output name=go-mod::$(go env GOMODCACHE)"
     - name: Go Build Cache
-      uses: actions/cache@v2
+      uses: actions/cache@v4
       with:
         path: ${{ steps.go-cache-paths.outputs.go-build }}
         key: ${{ runner.os }}-go-build-${{ hashFiles('**/go.sum') }}
     - name: Go Mod Cache
-      uses: actions/cache@v2
+      uses: actions/cache@v4
       with:
         path: ${{ steps.go-cache-paths.outputs.go-mod }}
         key: ${{ runner.os }}-go-mod-${{ hashFiles('**/go.sum') }}