Switch to libclang 11 for doxygen-generation step (#57)

Libclang 9 packages are no longer available on the latest Ubuntu (22.04).
Libclang 11 instead must be used.

See https://packages.ubuntu.com/search?keywords=libclang&searchon=names&suite=jammy&section=all

doxygen-generation/action.yml

@@ -26,7 +26,7 @@ inputs:
   doxygen_dependencies:
     description: 'Space-separated dependencies for doxygen.'
     required: false
-    default: libclang-9-dev libclang-cpp9 graphviz
+    default: libclang-11-dev libclang-cpp11 graphviz
 
 runs:
   using: "composite"