Support .NET 9

[martincostello]

Add support for .NET 9 and drop support for out-of-support versions of .NET.

Changes:

• Add net9.0 TFMs.
• Drop TFMs for all out-of-support versions of .NET (except .NET 6) to resolve Drop support for out-of-support .NET versions #2798.
• Bump version to 7.0.0.
• Only audit direct NuGet dependencies.
• Suppress vulnerabilities in IdentityServer4 used in sample flagged by the .NET 9 SDK.
• Remove some now-redundant pre-processor directives.
• Make all the middleware classes internal to resolve Consider making more types internal #2805.