THRIFT-5684 upgrade to net7.0:
Client: netstd
Patch: Jens Geyer
diff --git a/build/docker/old/debian-stretch/Dockerfile b/build/docker/old/debian-stretch/Dockerfile
index 628b626..25237ca 100644
--- a/build/docker/old/debian-stretch/Dockerfile
+++ b/build/docker/old/debian-stretch/Dockerfile
@@ -102,10 +102,10 @@
 # project isn't ready for this quite yet:
 # 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
 
 RUN apt-get install -y --no-install-recommends \
 `# Erlang dependencies` \