ci: bump actions/checkout to v6 and actions/setup-go to v6

Picked up from dependabot PRs #199 and #196 — superseded into the
lotus v1.36.0-rc1 release bundle.

Author: emmanuelm41

.github/workflows/main.yml

@@ -11,10 +11,10 @@ jobs:
       options: --tty
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           submodules: true
-      - uses: actions/setup-go@v5
+      - uses: actions/setup-go@v6
         with:
           go-version: '1.25.7'
       - name: Install dependencies
@@ -30,10 +30,10 @@ jobs:
     environment: DEVNET
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           submodules: true
-      - uses: actions/setup-go@v5
+      - uses: actions/setup-go@v6
         with:
           go-version: '1.25.7'
       - name: Install dependencies