manifest.yml: Remove node 10, replace with 12.x.x
manifest.yml: Remove node 10, replace with 12.x.x

See https://github.com/pivotal-cf/tanzu-buildpacks/issues/32

Co-authored-by: Arjun Sreedharan <29ce4cd6ea5d13cdc653e7ecc5dd8b599d496b8a@vmware.com>

cloudfoundry-community/asp.net5-buildpack