Update CI to .NET 6

.github/workflows/build.yml

@@ -3,8 +3,8 @@ name: Build
 on: [push, pull_request]
 
 env:
-  BUILD_DOTNET_VERSION: 5.0.x
-  BUILD_LINUX_TEST_TFM: net5.0
+  BUILD_DOTNET_VERSION: 6.0.x
+  BUILD_LINUX_TEST_TFM: net6.0
   DOTNET_NOLOGO: 1
 
 jobs: