updating workflow

Author: lemire

.github/workflows/amalgamate-ubuntu20.yml renamed to .github/workflows/amalgamate-ubuntu24.yml

@@ -1,10 +1,10 @@
-name: Amalgamate Ubuntu 20.04 CI (GCC 9)
+name: Amalgamate Ubuntu 20.04 CI (GCC 13æ)
 
 on: [push, pull_request]
 
 jobs:
   ubuntu-build:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-24.04
     steps:
       - uses: actions/checkout@v4
       - name: Compile with amalgamation