Don't run CI builds on Ubuntu 16 not supported any longer

Support for Ubuntu 16.04 ended on September 20, 2021.
See: https://github.blog/changelog/2021-04-29-github-actions-ubuntu-16-04-lts-virtual-environment-will-be-removed-on-september-20-2021/

See https://github.com/wxWidgets/wxWidgets/pull/2531

.github/workflows/ci.yml

@@ -49,19 +49,19 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - name: Ubuntu 16.04 wxGTK 2
-            runner: ubuntu-16.04
+          - name: Ubuntu 18.04 wxGTK 2
+            runner: ubuntu-18.04
             gtk_version: 2
             use_xvfb: true
-          - name: Ubuntu 16.04 wxGTK 3 static
-            runner: ubuntu-16.04
-            configure_flags: --disable-shared
-            use_xvfb: true
           - name: Ubuntu 18.04 wxGTK 2 UTF-8
             runner: ubuntu-18.04
             gtk_version: 2
             configure_flags: --enable-utf8 --enable-utf8only --enable-monolithic
             use_xvfb: true
+          - name: Ubuntu 18.04 wxGTK3 static
+            runner: ubuntu-18.04
+            configure_flags: --disable-shared
+            use_xvfb: true
           - name: Ubuntu 18.04 wxGTK 3 STL
             runner: ubuntu-18.04
             configure_flags: --enable-cxx11 --enable-stl --disable-compat30