thrift: switch csharp backend to netstd
thrift: switch csharp backend to netstd

The thrift compiler (since 0.13 at least) complains that
the csharp target is deprecated and recommend replacing it
with netstd. Since we don't use either, humor it.

I suspect that this warning caused some spurious rebuilds,
but have not proven it.
avikivity/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla