ilu0cgsolver.hh: use dune-istl's `SeqILU` instead of the deprecated `SeqILU0`

2 jobs for feature/compiler-warnings-and-deprecations in 3 minutes and 6 seconds (queued for 1 second)
Status Name Job ID Coverage
  Test
passed dune:git--clang #9587

00:03:02

passed dune:git--gcc #9588

00:03:06