THRIFT-5684 upgrade to net7.0:
Client: netstd
Patch: Jens Geyer
diff --git a/build/docker/old/ubuntu-xenial/Dockerfile b/build/docker/old/ubuntu-xenial/Dockerfile
index 7cb58e8..add524d 100644
--- a/build/docker/old/ubuntu-xenial/Dockerfile
+++ b/build/docker/old/ubuntu-xenial/Dockerfile
@@ -115,10 +115,10 @@
 
 RUN apt-get install -y --no-install-recommends \
       `# dotnet core dependencies` \
-      dotnet-sdk-6.0 \
-      dotnet-runtime-6.0 \
-      aspnetcore-runtime-6.0 \
-      dotnet-apphost-pack-6.0
+      dotnet-sdk-7.0 \
+      dotnet-runtime-7.0 \
+      aspnetcore-runtime-7.0 \
+      dotnet-apphost-pack-7.0
 
 # Erlang dependencies
 ARG ERLANG_OTP_VERSION=18.3.4.11