[cartservice] bump Otel .NET SDK to 1.8.1

[rogercoll]

Changes

Bump cartservice .NET dependencies to 1.8.1

Merge Requirements

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

• [x] 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.