Remove references to N1QL_SYSTEM_BUCKET

In versions 7.0, 7.1, and 7.2, when CBO is in effect and user issues UPDATE STATISTICS/ANALYZE command, the gathered optimizer statistics are stored in N1QL_SYSTEM_BUCKET.N1QL_SYSTEM_SCOPE.N1QL_CBO_STATS.

In version 7.6, the storage location has changed and instead the gathered optimizer statistics will now be stored in the _query collection under the _system scope, for each bucket. See MB-57717.

We need to update the documentation such that it use the proper location for optimizer statistics.