Patch release 1.15.0 with Istio 1.9.8

Big Bang release 1.15.0 has Istio 1.9.7. To get the latest security patches for our customers, we need to create a 1.15.1 release with Istio 1.9.8. The following activities need to be completed:

• Branch Istio Operator at last 1.9.7 release, update to 1.9.8, and create a tag. After tag is created, branch can be deleted since tag will prevent Git garbage collection on commit.
• Branch Istio Control Plane at last 1.9.7 release, update to 1.9.8, and create a tag. After tag is created, branch can be deleted since tag will prevent Git garbage collection on commit.
• Validate Istio is operating properly by connecting to applications through the ingress gateway
• Validate Authservice is working properly
• On Big Bang release branch for 1.15.0, update chart tags for Istio Operator and Istio Control Plane.
• Create a new release for Big Bang 1.15.1