Update CloudFormation template example and documentation to enable externally managed VPC and DNS

[ag-adampike]

A stack that is created using our current CloudFormation template example includes a VPC with public subnets and associated resources and DNS records using Route 53. This PR includes changes to enhance the template, enabling customers to:

• select an existing VPC and public subnets
• exclude the DNS records from the stack

Documentation has been updated accordingly, and includes additional changes to align with our other deployment examples.

Additional enhancements:

• updated TLS policy to enforce TLS 1.3
• refactored and simplified Redis configuration
• stripped redundant policy actions
• specified ARM64 platform to reduce expected cost
• stores unencoded values in AWS secrets
• writes secrets to volume instead of env var values
• reconfigured for zero-downtime rolling updates

Testing criteria:

• update an existing stack deployed using the existing template and no other changes
• create a new stack, specifying an existing VPC and subnets
• create a new stack without specifying a Route 53 hosted zone