Fix for retry cancellation (App-vNext#2456)

Fixes App-vNext#2375.

Synchronize access to Validator (App-vNext#2428)

Synchronize access to `System.ComponentModel.DataAnnotations.Validator`.

Fixes App-vNext#2412.

Update to .NET 9 SDK (App-vNext#2003)

- Build with the .NET 9 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.
- 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.
- Bump version to 8.5.0.

Downgrade sign tool

Downgrade the `sign` tool to the same version used when we published v8.4.1 to see if it's the source of the failure.

Handle nested inner exceptions (App-vNext#2166)

- Replicate behaviour of v7 policies and recursively check all inner exceptions and `AggregateException` to handle inner exceptions.
- Refactor predicate builder test cases to make a bit more readable.
- Use collection expression.

Rebase and cleanup the PR

Update CHANGELOG

Add changelog entries for Polly 8.3.1.

Update docs banners

Remove banners about upcoming features.

Co-Authored-By: peter-csala <57183693+peter-csala@users.noreply.github.com>

Add alt text to logo

Add alt text to .NET Foundation logo to resolve MD045 warning.

Add support for .NET 8 (App-vNext#1144)

- Add `net8.0` targets.
- Use .NET `TimeProvider` implementation.
- Use `FakeTimeProvider` for tests.
- Use new .NET 8 APIs where relevant.

Co-Authored-By: martintmk <103487740+martintmk@users.noreply.github.com>