ci: llvm19 -> llvm20.

This needs to be cherry-picked to master when we want to start testing the
llvm20 branch on the CI machines.

Author: alexrp

.github/workflows/ci.yaml

@@ -4,7 +4,7 @@ on:
   push:
     branches:
       - master
-      - llvm19
+      - llvm20
 concurrency:
   # Cancels pending runs when a PR gets updated.
   group: ${{ github.head_ref || github.run_id }}-${{ github.actor }}