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.

Will remain in draft until November 2024 when .NET 9 is officially released, with the intention to release soon afterwards

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.98%. Comparing base (6995d8a) to head (55ff025).
Report is 3 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3007      +/-   ##
==========================================
- Coverage   91.43%   81.98%   -9.46%     
==========================================
  Files          76       76              
  Lines        3119     3119              
  Branches      519      519              
==========================================
- Hits         2852     2557     -295     
- Misses        267      562     +295     

Flag	Coverage Δ
Linux	81.98% <100.00%> (-9.46%)	⬇️
Windows	81.98% <100.00%> (-9.46%)	⬇️
macOS	81.98% <100.00%> (-9.46%)	⬇️

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[bkoelman]

It's great to see preparations being made for .NET 9. I've shared some comments based on my own experience, that may be of help.

[martincostello]

@mesMess Could I get a review/approval on this PR ahead-of-time please? I'd like to be able to merge this and ship a release on 12th November once the stable 9.0.0 packages are in NuGet.org.

[jgarciadelanoceda]

@martincostello the dotnet6 release is going to be out of release the same day that dotnet9 comes, why not deleting also dotnet6?
Also if the nugget of STJ is greater than 8 we can delete a lot of compiled primitives that are present in #2988 have you considered it?

[martincostello]

It was considered (see original issue) but I decided against it to not generate a cliff on day 1 of support ending. 6 will be removed in a future release.

Keeping the package versions in line with the TFMs keeps things in configurations that the .NET team explicitly test for, makes things easier to reason about to support, and means assemblies that ship with the shared framework able to be referenced in the deployed application. Making a .NET 8 app use .NET 9 assemblies bloats the deployed application as it has to include a copy of the newer assemblies. You can see Aspire 9 does this for example for its net8.0 and net9.0 targets.

[jgarciadelanoceda]

Yeah I was thinking of .net standard and also framework only, because( I do not know if many are using), but it can give the users of that platforms the ability of using the new STJ attributes.
Just sifting the reference of the nugget to a recent one ( it was lower than 5).
The compile directive has to change in order to support this to net>7 or net standard.. hopefully that works

[martincostello]

Maybe, but sometimes libraries have some features that only light up in newer runtimes due to language features needed, or new APIs or overloads of them. Trying to run a .NET 6 app with a 9 package won't have been tested for, and won't necessarily be able to use any improvements (or worse, fall back to netstandard2.0 and then lose even 5.0 or 6.0 specific features).

Ultimately, it's not worth the complexity/burden to us in terms of support, over just using 6 with 6, 8 with 8 etc.

If people want new features, they can upgrade their apps to the version that introduces them.

[mesMess]

@mesMess Could I get a review/approval on this PR ahead-of-time please? I'd like to be able to merge this and ship a release on 12th November once the stable 9.0.0 packages are in NuGet.org.

Yep, I'll do that today.

[mesMess]

Build bump 7 -> 8 (next LTS)
Microsoft OenApi version bump
Bump to the RC for .NET 9
Moving some object to internal from public
Some of the shipped/un-shipped files are empty
Sealing some internal classes (UI)