there's no 20.04-arm, request the correct image for testing

Author: masklinn

.github/workflows/pyo3-wheels.yml

@@ -175,7 +175,7 @@ jobs:
 
           - runner: ubuntu-latest
           - arch: aarch64
-            runner: ubuntu-20.04-arm
+            runner: ubuntu-24.04-arm
           - platform: windows
             runner: windows-latest
           - platform: windows