Mark readonlyRootFilesystem as unsupported in CouchbaseCluster Resource

https://docs.couchbase.com/operator/current/resource/couchbasecluster.html#couchbaseclusters-spec-security-securitycontext-readonlyrootfilesystem

This is option is described as unsupported on Windows, but we've had confirmation from the Operator Team (see linked issue) that this is currently not supported across all environments, or at least not without altering some other configuration settings, which will take some work to figure out and document. It seems like at the moment we should not be recommending anyone sets this to true.

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Reporter: Jack Bakes
E-mail: jack.bakes@couchbase.com