chore: upgrade all GitHub Actions to latest versions

Node.js 20 is being deprecated in the Actions runtime. Upgrade every
action to its latest release so the workflows run on Node.js 22+:

  actions/checkout        v5/v6.0.2  → v7.0.0
  browser-actions/setup-chrome  v1   → v2.1.2
  golangci/golangci-lint-action v8   → v9.2.1
  goreleaser/goreleaser-action  v6   → v7.2.2
  docker/setup-qemu-action      v3   → v4.1.0
  docker/setup-buildx-action    v3   → v4.1.0
  docker/login-action           v3   → v4.2.0
  sigstore/cosign-installer     v3   → v4.1.2
  anchore/sbom-action           v0   → v0.24.0

.github/workflows/docs.yml

@@ -26,14 +26,14 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v6.0.2
+      - uses: actions/checkout@v7.0.0
         with:
           submodules: true
           # Sitemap lastmod comes from the latest content commit.
           fetch-depth: 0
 
       - name: Checkout tago
-        uses: actions/checkout@v6.0.2
+        uses: actions/checkout@v7.0.0
         with:
           repository: tamnd/tago
           path: .tago-src
@@ -107,7 +107,7 @@ jobs:
       group: cloudflare-pages-kage
       cancel-in-progress: true
     steps:
-      - uses: actions/checkout@v6.0.2
+      - uses: actions/checkout@v7.0.0
         with:
           fetch-depth: 1
           sparse-checkout: scripts/