doc: update release descriptions on packages page

Firefly was listed as the "latest release" of Ceph. Add Giant and label
it as the latest release instead.

Releases A-C and E are no longer supported. D will receive updates for
some time longer, and F is a "long term stable" release.

Signed-off-by: Ken Dreyer <kdreyer@redhat.com>
Merge pull request #2923 from ktdreyer/doc-install-giant

doc: update packages page for Giant

Reviewed-by: Loic Dachary <ldachary@redhat.com>