Detect earlier versions of dotnet

When using a version of dotnet that is too old, errors will occur. Instead verify dotnet version first.

No files or tests are named in the issue. Start by checking the repository’s build/setup files (solution/project and CI/config scripts) to find where dotnet is expected, then run the normal build command to reproduce the failure on an older runtime. Add an early version check in the same execution path so unsupported versions are rejected before later errors. Done means a clear failure about minimum dotnet version appears instead of cascading runtime/build errors.

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