Change references to net6.0 to net8.0

Author: lukebakken

CONTRIBUTING.md

@@ -119,7 +119,7 @@ Benchmarks can be found under `./projects/Benchmarks`. On the command line, run
 like so:
 
 ```
-dotnet run -c Release -f net6.0 --project ./projects/Benchmarks/ --filter Networking_BasicDeliver_LongLivedConnection
+dotnet run --configuration Release --project ./projects/Benchmarks/ --filter Networking_BasicDeliver_LongLivedConnection
 ```
 
 ## Code of Conduct

RUNNING_TESTS.md

@@ -127,8 +127,8 @@ dotnet test projects/Test/Unit.csproj --filter "FullyQualifiedName~RabbitMQ.Clie
 
 ## Running Tests for a Specific .NET Target
 
-To run tests targeting .NET 6.0:
+To run tests targeting .NET 8.0:
 
 ``` shell
-dotnet test --framework net6.0 projects/Unit
+dotnet test --framework net8.0 projects/Unit
 ```