We instruct users to update the CRDs when upgrading to 1.4 and 1.5 which
involves using `kubectl apply` to apply the CRD configuration. The CRD
configuration generated by `velero install` includes fields which are
not valid when running Kubernetes v1.14 or earlier. We instruct users to
work around this when doing a customised velero install, but not when
upgrading to newer versions. This change updates the upgrade
instructions for v1.4 and v1.5 to include the use of `--validate=false`
flag when running `kubectl apply`.

See #2077 and #2311 for more context.
Signed-off-by: Bridget McErlean <bmcerlean@vmware.com>