Removed .NET Core 3.1 and updated to .NET 6.0

[ckadluba]

• Removed soon to be obsolete .NET Core 3.1 target framework
• Support .NET 6.0 now instead
• Fixed .NET code analysis errors
• Cleaned dependencies in sample projects

[ckadluba]

@jonorossi Thank you for approving. I still need to fix a problem with the unit tests on .NET 6. Attempting to run them stops with the following error.

Starting test execution, please wait...
A total of 1 test files matched the specified pattern.
Testhost process exited with error: Error:
  An assembly specified in the application dependencies manifest (Serilog.Sinks.MSSqlServer.Tests.deps.json) was not found:
    package: 'Azure.Core', version: '1.6.0'
    path: 'lib/netstandard2.0/Azure.Core.dll'
. Please check the diagnostic logs for more information.

Under investigation. ;)