MemoryCache: obsolete API removed in 3.0

## MemoryCache: obsolete API removed in 3.0

This is a follow up to https://github.com/aspnet/Caching/issues/221. The 3.0 release will remove the [obsolete `MemoryCacheOptions` APIs](https://github.com/aspnet/Extensions/blob/dc5c593da7b72c82e6fe85abb91d03818f9b700c/src/Caching/Memory/src/MemoryCacheOptions.cs#L17-L18). 

Discussion: https://github.com/aspnet/Extensions/issues/1062. 

### Version introduced

3.0

### Old behavior

`MemoryCacheOptions.CompactOnMemoryPressure` property was available.

### New behavior

`MemoryCacheOptions.CompactOnMemoryPressure` has been removed.

### Reason for change

Automatically compacting the cache caused problems. We recommend compacting the cache when needed to avoid unexpected behavior.

### Recommended action

To compact, downcast to `MemoryCache` and call `Compact` when needed.

### Category

- ASP.NET Core

### Affected APIs

[MemoryCacheOptions.CompactOnMemoryPressure](https://docs.microsoft.com/en-us/dotnet/api/microsoft.extensions.caching.memory.memorycacheoptions.compactonmemorypressure?view=aspnetcore-2.2)



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#### Issue metadata

* Issue type: breaking-change

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MemoryCache: obsolete API removed in 3.0 #14772