pythongh-140889: Bump tailcall and JIT CI to llvm 20 (python#140963)

Author: Fidget-Spinner

.github/workflows/jit.yml

@@ -193,7 +193,7 @@ jobs:
       fail-fast: false
       matrix:
         llvm:
-          - 19
+          - 20
     steps:
       - uses: actions/checkout@v4
         with: