ci: upgrade actions to Node.js 24-compatible versions

Node.js 20 actions are deprecated and will be forced to Node.js 24
starting June 2, 2026. Bump to versions that ship Node.js 24 runtimes:

  actions/checkout        v4 → v6
  actions/setup-go        v5 → v6
  golangci-lint-action    v7 → v9
  actions/upload-artifact v4 → v6
  actions/download-artifact v4 → v8

.github/workflows/go.yml

@@ -46,10 +46,10 @@ jobs:
           --mount type=tmpfs,destination=/var/lib/postgresql/data
 
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
 
     - name: Set up Go
-      uses: actions/setup-go@v5
+      uses: actions/setup-go@v6
       with:
         go-version: '1.25'
 
@@ -80,10 +80,10 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
 
     - name: Set up Go
-      uses: actions/setup-go@v5
+      uses: actions/setup-go@v6
       with:
         go-version: '1.25'
 
@@ -94,7 +94,7 @@ jobs:
       run: env GOOS=linux GOARCH=amd64 go build -v
 
     - name: Upload Linux-amd64 artifacts
-      uses: actions/upload-artifact@v4
+      uses: actions/upload-artifact@v6
       with:
         name: Linux-amd64
         path: |
@@ -109,7 +109,7 @@ jobs:
       run: env GOOS=windows GOARCH=amd64 go build -v
 
     - name: Upload Windows-amd64 artifacts
-      uses: actions/upload-artifact@v4
+      uses: actions/upload-artifact@v6
       with:
         name: Windows-amd64
         path: |
@@ -125,15 +125,15 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
 
     - name: Set up Go
-      uses: actions/setup-go@v5
+      uses: actions/setup-go@v6
       with:
         go-version: '1.25'
 
     - name: Run golangci-lint
-      uses: golangci/golangci-lint-action@v7
+      uses: golangci/golangci-lint-action@v9
       with:
         version: v2.10.1
         args: --timeout=5m