Upgrade to .NET 6 RC2

[dominik-oktav]

Fix compilation errors:

• Expose internals of DistributedResponseCachingMiddleware to DistributedResponseCachingMiddlewareTests. This fixes Improved MS reference from new code #5.
• Consider new namespace of CachedResponse

Upgradef to NET 6 RC2:

• Set target framework to net6.0
• Updated nuget packages to 6.0 or latest
• Set Version to 6.0.0-rc2

Bugfixes:

• Register ResponseCachingPolicyProvider as IResponseCachingPolicyProvider when CacheAuthorizedRequest is not enabled, to fix error when resolving for IResponseCachingPolicyProvider
• Fix configuration of ResponseCachingOptions by configuring it explicitly. By just registering the CustomResponseCachingOptions, the options of the base objects would get lost when resolving for IOptions.

[yringler]

This looks great, thank you!
Merged, will try to update the nuget package later today.
Would you be able to update the documentation (Readme) also?

Also, I'm not using this package; it would be better if it was maintained by someone who's using it.
Would you be interested in being added as maintainer?
I could even see about having you able to upload builds to nuget.

[dominik-oktav]

Thanks for the offer, I gladly would be maintainer of this project. This would make it much easier for us to make future changes if necessary.