ci: upgrade GitHub Actions to v6 for Node.js 24 compatibility

actions/checkout and actions/setup-node v4 run on Node.js 20 which is
deprecated and will be removed from runners on September 16th, 2026.

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

Author: brettstack

.github/workflows/release-on-push-to-mainline.yml

@@ -13,10 +13,10 @@ jobs:
         node-version: [22.x, 24.x]
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Use Node.js ${{ matrix.node-version }}
-        uses: actions/setup-node@v4
+        uses: actions/setup-node@v6
         with:
           node-version: ${{ matrix.node-version }}
 
@@ -48,10 +48,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Use Node.js 24.x
-        uses: actions/setup-node@v4
+        uses: actions/setup-node@v6
         with:
           node-version: 24.x
       - name: Install