[checkout] update grpc clients

[puckpuck]

Changes

Updates Go gRPC client calls to move away from deprecated DialContext in favor of NewClient call.

The productcatalogservice also had code for this call, but it was in an unused function, so I just removed that function. This makes sense, since productcatalog doesn't make any client calls.

Merge Requirements

For new features contributions please make sure you have completed the following
essential items:

• CHANGELOG.md updated to document new feature additions
  * [ ] Appropriate documentation updates in the docs
  * [ ] Appropriate Helm chart updates in the helm-charts

Maintainers will not merge until the above have been completed. If you're unsure
which docs need to be changed ping the
@open-telemetry/demo-approvers.