- Add `net9.0` TFMs.
- Drop TFMs for all out-of-support versions of .NET.
- Bump version to `7.0.0`.
- Only audit direct NuGet dependencies.
- Suppress vulnerabilities in IdentityServer4 used in sample.
- Remove some now-redundant pre-processor directives.

Target .NET 9 versions for Microsoft.AspNetCore.OpenApi in test sites.

Bump Microsoft.OpenApi to the latest version.

* Update .NET SDK

Update .NET SDK to version 9.0.100-preview.7.24407.12.

---
updated-dependencies:
- dependency-name: Microsoft.NET.Sdk
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

* Update NuGet packages
Bump NuGet packages for .NET 9 preview 7.

---------

Signed-off-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Martin Costello <[email protected]>

Bump Microsoft.OpenApi to 1.6.18.

Bump Microsoft.OpenApi to 1.6.20.

Update Microsoft.OpenApi to 1.6.21.

* Update .NET SDK

Update .NET SDK to version 9.0.100-rc.1.24452.12.

---
updated-dependencies:
- dependency-name: Microsoft.NET.Sdk
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

* Update NuGet packages
Update NuGet packages for RC1.

---------

Signed-off-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Martin Costello <[email protected]>

Make all the middleware classes internal as they have no useful public-facing functionality (e.g. virtual members).

Resolves #2805.

* Update .NET SDK

Update .NET SDK to version 9.0.100-rc.2.24474.11.

---
updated-dependencies:
- dependency-name: Microsoft.NET.Sdk
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

* Update NuGet packages
Update NuGet packages for .NET 9 RC2.

---------

Signed-off-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Martin Costello <[email protected]>

Bump Microsoft.OpenApi to 1.6.22.

Update packages for ASP.NET Core 8.0.10.

Put the newest first so Visual Studio loads the latest version as the selected TFM by default.

- Remove redundant comment.
- Add link to tracking issue in comment.