SignalR redis-backplane: Needs update and inline code moved to new app sample

### Description

This topic refers to startup config and the code is in line to the topic, so it would be good to move that out into a complete 8.0 app sample. I won't be able to provide Redis with it, so it will have to be a working example I tested with Redis, but just includes the app not the db.

Related to #31631, a community member may update "MyApp" to RedisChannel.Literal("MyApp") which I will keep an eye on.

### Page URL

https://learn.microsoft.com/en-us/aspnet/core/signalr/redis-backplane?view=aspnetcore-8.0

### Content source URL

https://github.com/dotnet/AspNetCore.Docs/blob/main/aspnetcore/signalr/redis-backplane.md

### Document ID

bddd7575-422d-4342-af48-f6b0979fb69a

### Article author

@bradygaster

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[Associated WorkItem - 226928](https://dev.azure.com/msft-skilling/Content/_workitems/edit/226928)