[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.