[CI] Uplift Ubuntu version for GH pages workflow.

This should fix the installation of GitHub Actions dependencies broken by recent updates.
See #12498 (comment) for more details.

Author: bader

.github/workflows/gh_pages.yml

@@ -13,7 +13,7 @@ on:
 
 jobs:
   build:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     if: github.repository == 'intel/llvm'
     steps:
     - uses: actions/checkout@v4