Adding information to arcadesdk.md to explain why we specify dotnet.exe properties to target a matching .Net Core SDK version in servicing branches.

[davfost]

Adding information to arcadesdk.md to explain why we specify dotnet.exe properties to target a matching .Net Core SDK version in servicing branches.

To double check:

• [x] The right tests are in and and the right validation has happened. Guidance: https://github.com/dotnet/arcade/tree/main/Documentation/Validation