build: bump actions/checkout to v5

v4 runs on Node 20, which GitHub is forcing off the runners in
September 2026. v5 runs on Node 24 and is a drop-in replacement.
Surfaced as a warning annotation on the CI modernization run.

Author: eiennohito

.github/workflows/cmake.yml

@@ -52,7 +52,7 @@ jobs:
         shell: bash
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v5
 
       - name: Install dependencies (Ubuntu)
         if: startsWith(matrix.os, 'ubuntu-')