Add a CI build using C++20

This should allow detecting C++20-specific problems, such as warnings
about combining incompatible enums.

.github/workflows/ci.yml

@@ -88,6 +88,10 @@ jobs:
             skip_samples: true
             use_asan: true
             use_xvfb: true
+          - name: Ubuntu 22.04 wxGTK C++20
+            runner: ubuntu-22.04
+            configure_flags: --with-cxx=20
+            skip_samples: true
           - name: Ubuntu 18.04 wxX11
             runner: ubuntu-18.04
             configure_flags: --with-x11 --enable-pch --disable-stc