This is another attempt to make clang-8 tests not fail in C++20 mode. Remove
gcc-10 and libstdc++-10 to prevent clang using it.
Also, use apt-get instead of apt since the former is recommended to use in
scripts. This should silence apt warnings.
clang-8 jobs fail in C++20 mode since it doesn't support
std::is_constant_evaluated, which is called from libstdc++-10 that is installed
in Focal image. Moving it back to Bionic should work around the issue
as it should use an older libstdc++ version.
gcc-10 and 11 were moved to Focal just to make this configuration tested.
Also, changed 2a to 20 for compilers that support this.
