[ci] Use ubuntu version images in the CI from 20.04 to 22.04.

What this MR does / why does DuMux need it:
See issue #1316 (closed)

test/porousmediumflow/2p/incompressible/main.cc

@@ -39,6 +39,8 @@
 #define DIFFMETHOD DiffMethod::numeric
 #endif
 
+//fake change for CI
+
 int main(int argc, char** argv)
 {
     using namespace Dumux;