support for rocky linux

Hi,
since rocky linux seems to replace centos, as centos has changed its roadmap, it will be really helpful to add rocky linux to curtin distro:

stdout: start: cmd-install/stage-curthooks/builtin/cmd-curthooks: curtin command curthooks
        Running curtin builtin curthooks
        finish: cmd-install/stage-curthooks/builtin/cmd-curthooks: FAIL: curtin command curthooks
        Traceback (most recent call last):
          File "/curtin/curtin/commands/main.py", line 202, in main
            ret = args.func(args)
          File "/curtin/curtin/commands/curthooks.py", line 1891, in curthooks
            builtin_curthooks(cfg, target, state)
          File "/curtin/curtin/commands/curthooks.py", line 1686, in builtin_curthooks
            distro_info = distro.get_distroinfo(target=target)
          File "/curtin/curtin/distro.py", line 118, in get_distroinfo
            variant = name_to_distro(variant_name)
          File "/curtin/curtin/distro.py", line 57, in name_to_distro
            return DISTROS[DISTROS.index(distname)]
        ValueError: tuple.index(x): x not in tuple
        tuple.index(x): x not in tuple

Stderr: ''

distro.py should have support to rocky linux

BR
Francesco