Remove dotnet 3.1 from makefile.

Makefile

@@ -30,7 +30,7 @@ php-browser-test: php-browser-test_8.2 php-browser-test_latest
 mobile: android-build-server
 static-code-analysis: static-code-analysis_latest static-code-analysys_php static-code-analysys_python static-code-analysys_android
 utility: alpine-ssh-client merge-request-check
-dotnet: dotnet_3.1 dotnet_6.0
+dotnet: dotnet_6.0
 
 # PHP Lint Images
 ####################
@@ -144,9 +144,6 @@ dependency-check: dependency-check/Dockerfile
 
 # dotnet Images
 ####################
-dotnet_3.1: dotnet-build-server/3.1/Dockerfile
-	docker build $(BUILD_ARGS) -t ${REPO_ORG}/dotnet-build-server:3.1 -f dotnet-build-server/3.1/Dockerfile dotnet-build-server/3.1/
-
 dotnet_6.0: dotnet-build-server/6.0/Dockerfile
 	docker build $(BUILD_ARGS) -t ${REPO_ORG}/dotnet-build-server:6.0 -f dotnet-build-server/6.0/Dockerfile dotnet-build-server/6.0/