- Build with the .NET 9 (preview 6) SDK.
- Add tests TFM for `net9.0`.
- Remove tests for `net6.0` TFM.
- Run workflows on PRs to `dotnet-vnext` branch.
- Only audit direct NuGet dependencies.
- Fix IDE0022 warning.
- Temporarily disable some tests that are failing in GitHub Actions.

Disable TFM parallelism for `dotnet test` in the Polly.Core project on Windows to fix two failing tests.
See https://learn.microsoft.com/dotnet/core/whats-new/dotnet-9/sdk#run-tests-in-parallel.

Set the coverage threshold back to 100%.

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: polly-updater-bot[bot] <138034000+polly-updater-bot[bot]@users.noreply.github.com>
Co-authored-by: polly-updater-bot[bot] <138034000+polly-updater-bot[bot]@users.noreply.github.com>

Update NuGet packages for .NET 9 preview 7.

Update Microsoft.Extensions.TimeProvider.Testing to .NET 9 preview 7.

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: polly-updater-bot[bot] <138034000+polly-updater-bot[bot]@users.noreply.github.com>
Co-authored-by: polly-updater-bot[bot] <138034000+polly-updater-bot[bot]@users.noreply.github.com>

Update NuGet packages for RC1.

Update the TimeProvider testing package to preview 8.

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: polly-updater-bot[bot] <138034000+polly-updater-bot[bot]@users.noreply.github.com>
Co-authored-by: polly-updater-bot[bot] <138034000+polly-updater-bot[bot]@users.noreply.github.com>

Bump NuGet packages for .NET 9 RC 2.

- Add usage of `[DebuggerDisableUserUnhandledExceptions]` to avoid newer versions of the Visual Studio debugger for breaking for exceptions on async code paths we are intentionally handling.
- Add copy of `[DebuggerDisableUserUnhandledExceptions]` for use in downlevel versions of .NET that do not contain the attribute.

- Update to the stable version of .NET 9.
- Bump version to 8.5.0.