[check.yml] Update CI to actions/checkout@v6 and actions/upload-artifact@v7 (cplusplus#9043)

The old versions use a deprecated Node.js version.

Author: eisenwave

.github/workflows/check.yml

@@ -22,7 +22,7 @@ jobs:
 
     steps:
       - name: checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: install GNU tools
         if: matrix.cfg.os == 'macos-15'
@@ -67,7 +67,7 @@ jobs:
 
       - name: upload PDF
         if: matrix.cfg.os == 'ubuntu-24.04'
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v7
         with:
           name: draft-snapshot
           path: source/std.pdf