ci: bump actions to Node 24-native versions

GitHub forces Node 20 actions to run on Node 24 starting 2026-06-02
and removes Node 20 entirely on 2026-09-16. Bump every action that
emitted the deprecation warning to the latest major that natively
declares runs.using: node24:

  actions/checkout                 v4 -> v6
  actions/setup-go                 v5 -> v6
  aws-actions/setup-sam            v2 -> v3
  aws-actions/configure-aws-credentials v4 -> v6

Verified action.yml runtime declarations (node24) for checkout v6
and setup-go v6. configure-aws-credentials v5 changed only invalid-
boolean-input handling, which we don't pass. setup-sam v3 is solely
the Node bump. golangci-lint-action stays at v7 (already Node 24,
never appeared in the deprecation warning).

.github/workflows/ci.yml

@@ -16,9 +16,9 @@ jobs:
       matrix:
         go: ['1.25']
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
-      - uses: actions/setup-go@v5
+      - uses: actions/setup-go@v6
         with:
           go-version: ${{ matrix.go }}
           cache: true
@@ -69,8 +69,8 @@ jobs:
     name: SAM template validate
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
-      - uses: aws-actions/setup-sam@v2
+      - uses: actions/checkout@v6
+      - uses: aws-actions/setup-sam@v3
         with:
           use-installer: true
       - name: sam validate (offline)