[ci]Update CI toolchains

Carsten Gräserrequested to merge
feature/modernize-ci-toolchain-after-2.9 into master
  • Use images based on debian 11 and debian 10. (Unfortunately there's no up-to date image dune:git-debian-11-clang... yet.)
  • Test against 2.9 release instead of 2.8

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