automation: Remove clang-8 from Debian unstable container

automation: Remove clang-8 from Debian unstable container

First, apt complain that it isn't the right way to add keys anymore,
but hopefully that's just a warning.

Second, we can't install clang-8:
The following packages have unmet dependencies:
 clang-8 : Depends: libstdc++-8-dev but it is not installable
           Depends: libgcc-8-dev but it is not installable
           Depends: libobjc-8-dev but it is not installable
           Recommends: llvm-8-dev but it is not going to be installed
           Recommends: libomp-8-dev but it is not going to be installed
 libllvm8 : Depends: libffi7 (>= 3.3~20180313) but it is not installable
E: Unable to correct problems, you have held broken packages.

clang on Debian unstable is now version 14.0.6.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>