Update to Supported Platforms Page for .NET 9 RC1 (#6855)

Author: poornimanayar

nservicebus/upgrades/supported-platforms.md

@@ -21,6 +21,7 @@ Support policies for individual NServiceBus components can be found in the [NSer
 |------------------|:-------:|:--------:|:-------:|
 | .NET | 8.0 (LTS) | [Windows / Linux](https://github.com/dotnet/core/blob/main/release-notes/8.0/supported-os.md) | [Supported until November 10, 2026](https://dotnet.microsoft.com/en-us/platform/support/policy/dotnet-core) |
 | .NET | 9.0 (preview 6) | [Windows / Linux](https://github.com/dotnet/core/blob/main/release-notes/9.0/supported-os.md) | Supported, for extra information view [the announcement](https://discuss.particular.net/t/nservicebus-for-net-9-preview/4036)  |
+| .NET | 9.0 (RC 1) | [Windows / Linux](https://github.com/dotnet/core/blob/main/release-notes/9.0/supported-os.md) | Supported, for extra information view [the announcement](https://discuss.particular.net/t/nservicebus-for-net-9-release-candidate-1/4132)  |
 
 macOS is supported for development purposes but not for production workloads.