qa/suites/rados/encoder: remove rocky from supported distros

qa/suites/rados/encoder: remove rocky from supported distros

The encoder test tries to download reef and squid packages, but
these do not support rocky10. So, we should remove that distro
from the supported distros- only distros that all the involved
releases have in common should be tested.

Rocky10 will only be backported as far as tentacle.

Fixes: https://tracker.ceph.com/issues/75763
Signed-off-by: Laura Flores <lflores@ibm.com>