Upgrade guide should remove latest tag for image streams

During the 2.8 -> 2.9 upgrade the user is instructed to move away from the 'latest' tag for the various images and use a specific version. However it does not instruct the user to remove or update the `latest` tag itself. This results in the deployment being in a state that has caused some confusion where the 'latest' tag is always pointing at version '2.8'.

The following sections should include steps to remove the latest tag from the image streams all together to avoid this confusion:

https://access.redhat.com/documentation/en-us/red_hat_3scale_api_management/2.9/html/migrating_3scale/upgrade-template#patch_the_literal_system_literal_image

https://access.redhat.com/documentation/en-us/red_hat_3scale_api_management/2.9/html/migrating_3scale/upgrade-template#patch_the_literal_apicast_literal_image

https://access.redhat.com/documentation/en-us/red_hat_3scale_api_management/2.9/html/migrating_3scale/upgrade-template#patch_the_literal_backend_literal_image

https://access.redhat.com/documentation/en-us/red_hat_3scale_api_management/2.9/html/migrating_3scale/upgrade-template#patch_the_literal_zync_literal_image

https://access.redhat.com/documentation/en-us/red_hat_3scale_api_management/2.9/html/migrating_3scale/upgrade-template#patch_the_literal_system_memcached_literal_image

https://access.redhat.com/documentation/en-us/red_hat_3scale_api_management/2.9/html/migrating_3scale/upgrade-template#patch_the_literal_zync_database_postgresql_literal_image

https://access.redhat.com/documentation/en-us/red_hat_3scale_api_management/2.9/html/migrating_3scale/upgrade-template#backend-redis

https://access.redhat.com/documentation/en-us/red_hat_3scale_api_management/2.9/html/migrating_3scale/upgrade-template#system-redis

https://access.redhat.com/documentation/en-us/red_hat_3scale_api_management/2.9/html/migrating_3scale/upgrade-template#system-mysql

https://access.redhat.com/documentation/en-us/red_hat_3scale_api_management/2.9/html/migrating_3scale/upgrade-template#system-postgresql