Post release tasks

After releasing Octopus 13.0, we need to take care of the following tasks:

• Remove support for libxc 4.X
• Add support for libxc 6.X
• Remove cg_new
• Remove deprecation warning
• Fix m4 to forbid libxc4
• Clean the m4 to not test libxc4
• Fix the mk_functional_list.pl script
• Remove libxc4/libxc5 branches in test files
• Update distcheck builder 2021a -> 2022a (libxc4 -> libxc5)