Add dotnet 9.0 support

[EEParker]

This simply adds support for net9.0 in the nuget package.

NOTE: CodeQL does not have net9.0 support yet. This will stay pre-release until that dependency is met.

[github-actions[bot]]

Package	Line Rate	Branch Rate	Health
Serilog.Sinks.Splunk.TCP	65%	43%	➖
Serilog.Sinks.Splunk	19%	19%	❌
Serilog.Sinks.Splunk	50%	63%	❌
Summary	43% (334 / 812)	44% (86 / 208)	❌

[VictorioBerra]

It just came out today! You're fast!