Update Dockerfile

Updated to 1.21.0

Author: skaface-443

Dockerfile

@@ -2,7 +2,7 @@ FROM mcr.microsoft.com/dotnet/runtime:7.0
 
 # Env var
 ENV SERVER_BRANCH="stable" \
-    SERVER_VERSION="1.20.12" \
+    SERVER_VERSION="1.21.0" \
     SERVER_PORT="42420" \
     WORLDCONFIG_PROPICK_NODE_SEARCH_RADIUS="6" \
     UID="1000" \