Upgrade .NET 8-targeted projects to .NET 10 before end of support

## Context

.NET 8 reaches end of support on November 10, 2026:
https://dotnet.microsoft.com/en-us/platform/support/policy/dotnet-core

Several projects, examples, tests, and Docker images in this repository currently target or use .NET 8.

## Proposal

Consider upgrading the .NET 8-targeted projects and Docker build/runtime images to .NET 10 before .NET 8 reaches end of support.

The core `conductor-csharp` project currently targets `netstandard2.0`, so it may not need a target-framework change. It would be useful to confirm the intended compatibility policy for that package separately.

Inspect the repository's project, example, test, and Docker build/runtime image definitions to find every .NET 8 reference. Confirm the compatibility policy for the core conductor-csharp netstandard2.0 package separately, then run the existing test suite. Done means the intended .NET 8 targets and images use .NET 10, with compatibility decisions documented.

Written by the indexing model from the issue text.