Distributed caches are always mem caches

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Is your feature request related to a problem? Please describe.

\OCP\ICacheFactory::createDistributed returns a \OCP\ICache. In practice the objects are always \OCP\IMemcache, similar to createLocking results.

From a static analysis point of view you can't rely on that assumption.

Describe the solution you'd like

Finish https://github.com/nextcloud/server/pull/30072 so the interface returns a more specific type. This is not a backwards compatible change as any code that expected an ICache will still work.

Describe alternatives you've considered

N/a

Additional context

N/a

Start by reviewing the OCP\ICacheFactory::createDistributed and createLocking entry points, then inspect pull request 30072. Compare the declared return types with the concrete cache interfaces and check the relevant static-analysis or interface tests. Done means the distributed-cache return type is more specific without breaking existing callers.