Drop net6.0 from TargetFrameworks (#3208)

Experiment to reduce package size for the MyGet size limit. Consumers
on net6.0 fall back to the netstandard2.0 asset via NuGet compatibility,
losing the UNIX_SOCKET code path and other net8.0+ optimizations without
a build-time signal.

Reduces the StackExchange.Redis package from ~6.29MB to ~5.26MB (-16.5%).