[ci]Stop using images with debian-10 (oldstable)

* Use `dune:2.8-debian-11-gcc-10-20` for testing against the 2.8 release
* Use `dune:git-ubuntu-20.04-clang-10-20` for testing againt master with clang.
  This helps to avoid an issue with clang-7 and boost-serilization in debian-10.