THRIFT-5832 Drop net6 support and add net9 instead
Client: netstd
Patch: Jens Geyer
diff --git a/build/docker/ubuntu-focal/Dockerfile b/build/docker/ubuntu-focal/Dockerfile
index 84bbee1..02f84c6 100644
--- a/build/docker/ubuntu-focal/Dockerfile
+++ b/build/docker/ubuntu-focal/Dockerfile
@@ -139,10 +139,10 @@
 
 RUN apt-get install -y --no-install-recommends \
       `# dotnet core dependencies` \
-      dotnet-sdk-8.0 \
-      dotnet-runtime-8.0 \
-      aspnetcore-runtime-8.0 \
-      dotnet-apphost-pack-8.0
+      dotnet-sdk-9.0 \
+      dotnet-runtime-9.0 \
+      aspnetcore-runtime-9.0 \
+      dotnet-apphost-pack-9.0
 
 # Erlang dependencies
 ARG ERLANG_OTP_VERSION=23.3.4.11