Address compiler warnings, update SDK

[jviau]

This PR addresses compile warnings, getting the build down to 0 warnings.

• Updates SDK to latest .net 9 SDK
• Updates test and sample projects all to net8.0 (net6.0 is out of support).
  • There are 2 public packages which use net6.0. Dropping net6.0 support should be considered. Azure SDK and functions SDK are dropping support in a minor version rev based on dotnet teams guidance (even though it could be considered a 'breaking change', they have decided to not consider breaking out of support versions as important)
• Code changes to address warnings, either through applying recommendations or suppressing