doc/install: Update EOL version floor references in index.rst

The install overview cited Octopus (EOL 2022) as the cephadm minimum
and Nautilus (EOL 2021) as the Rook minimum. Update to Reef and Pacific
respectively, which are the currently supported release floor values.
Remove the repeated Nautilus/Octopus codename references in the
ceph-ansible paragraph, as these are historical release names not
needed to convey the message.

Fixes: https://tracker.ceph.com/issues/77192

Signed-off-by: Emmanuel Ameh <eameh@contractor.linuxfoundation.org>