qa/rgw/upgrade/pacific: remove centos_8.stream.yaml and rely on ubuntu_20.04.yaml

qa/rgw/upgrade/pacific: remove centos_8.stream.yaml and rely on ubuntu_20.04.yaml

we can't test this pacific->reef upgrade path on centos because pacific doesn't
have centos 9 builds, and reef no longer has centos 8 builds. only test
this upgrade on ubuntu focal which is still supported for both releases

this commit targets the reef branch directly because this rgw/upgrade/pacific
suite no longer exists on main and squid branches

Signed-off-by: Casey Bodley <cbodley@redhat.com>