Use ubuntu-26.04 runners in all GitHub workflows

Install clang-19 from the Ubuntu archive, as apt.llvm.org has no LLVM 19 repository for 26.04.

.github/workflows/close-label.yml

@@ -7,7 +7,7 @@ jobs:
   label_issues:
     # Skip issues closed as "not planned" or "duplicate"
     if: github.event.issue.state_reason == 'completed'
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-26.04
     permissions:
       issues: write
     steps: