update nanoserver docker image

Author: ManickaP

eng/docker/libraries-sdk-aspnetcore.windows.Dockerfile

@@ -1,6 +1,6 @@
 # escape=`
 # Simple Dockerfile which copies library build artifacts into target dotnet sdk image
-ARG SDK_BASE_IMAGE=mcr.microsoft.com/dotnet/nightly/sdk:6.0-nanoserver-1809
+ARG SDK_BASE_IMAGE=mcr.microsoft.com/dotnet/nightly/sdk:6.0-nanoserver-20H2
 FROM $SDK_BASE_IMAGE as target
 
 ARG TESTHOST_LOCATION=".\\artifacts\\bin\\testhost"

eng/docker/libraries-sdk.windows.Dockerfile

@@ -1,6 +1,6 @@
 # escape=`
 # Simple Dockerfile which copies clr and library build artifacts into target dotnet sdk image
-ARG SDK_BASE_IMAGE=mcr.microsoft.com/dotnet/nightly/sdk:6.0-nanoserver-1809
+ARG SDK_BASE_IMAGE=mcr.microsoft.com/dotnet/nightly/sdk:6.0-nanoserver-20H2
 FROM $SDK_BASE_IMAGE as target
 
 ARG TESTHOST_LOCATION=".\\artifacts\\bin\\testhost"

src/libraries/System.Net.Http/tests/StressTests/HttpStress/windows.Dockerfile

@@ -1,5 +1,5 @@
 # escape=`
-ARG SDK_BASE_IMAGE=mcr.microsoft.com/dotnet/nightly/sdk:6.0-nanoserver-1809
+ARG SDK_BASE_IMAGE=mcr.microsoft.com/dotnet/nightly/sdk:6.0-nanoserver-20H2
 FROM $SDK_BASE_IMAGE
 
 # Use powershell as the default shell

src/libraries/System.Net.Security/tests/StressTests/SslStress/windows.Dockerfile

@@ -1,5 +1,5 @@
 # escape=`
-ARG SDK_BASE_IMAGE=mcr.microsoft.com/dotnet/nightly/sdk:6.0-nanoserver-1809
+ARG SDK_BASE_IMAGE=mcr.microsoft.com/dotnet/nightly/sdk:6.0-nanoserver-20H2
 FROM $SDK_BASE_IMAGE
 
 # Use powershell as the default shell