Update blue-green deployment docs to not reference removed -n option

[beyhan]

The blue-green deployment docs mention cf push -n where the --hostname, -n option is removed with the CF CLI V7 as documented in the Upgrading to cf CLI v7. The upgrade guide mentions [Removed flag]: --hostname, -n. You can use the routes property in the manifest instead.

[anita-flegg]

@pspinrad

[pspinrad]

Thanks @beyhan -- pasting some related old notes I have, just to put them here, glad you brought this up:
To change: CLI v6 relics:
cf push -n flag not supported for v7/v8. Set hostname via routes property in manifest.
cf map-route --hostname option not supported for v7/v8. Set hostname via routes property in manifest.
cf update-buildpack BUILDPACK --rename option not supported. Use cf rename-buildpack instead.