HCP cluster upgrade from 4.18.26 to 4.19.z fails with error `router load balancer is not provisioned: Failed to allocate IP for "[namespace-namespace]/router": no available IPs`

Issue

• HCP cluster upgarde from 4.18.26 to 4.19.z fails immediately after triggering the upgrade from ACM UI.
• The upgrade fails with following error.

router load balancer is not provisioned: Failed to allocate IP for "<cluster_name>-<cluster_name>/router": no available IPs

• A service of type Loadbalencer is created with the name router. This service fails to get an IP address.
• Attempts to login into the hosted cluster fail with following errors. Also, the ingress operator is degraded after the upgrade is triggered.

Error from server (InternalError): Internal error occurred: unexpected response: 503