Update Travis .NET version (confluentinc#1931)

* Update Travis .NET version

* Fix makefile

Author: anchitj

.travis.yml

@@ -8,12 +8,12 @@ matrix:
   include:
   - os: linux
     dist: focal # Ubuntu 20.04
-    dotnet: 5.0.408
+    dotnet: 6.0.403
     mono: none
     sudo: required
   - os: osx
     osx_image: xcode9.4 # macOS 10.13
-    dotnet: 5.0.408
+    dotnet: 6.0.403
     mono: none
 
 before_install:

Makefile

@@ -10,7 +10,7 @@ EXAMPLE_DIRS=$(shell find ./examples -name '*.csproj' -exec dirname {} \;)
 TEST_DIRS=$(shell find ./test -name '*.csproj'   \;)
 UNIT_TEST_DIRS=$(shell find . -type d -regex '.*UnitTests$$' -exec basename {} \;)
 
-DEFAULT_FRAMEWORK?=net5.0
+DEFAULT_FRAMEWORK?=net6.0
 
 all:
 	@echo "Usage:   make <dotnet-command>"