Add last index scan time to system:indexes

Index last scan time has been exposed in system:indexes. We need to update the doc link: https://docs.couchbase.com/server/current/n1ql/n1ql-intro/sysinfo.html Section: Query Indexes

Insert this paragraph between keyspace_id and name

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metadata.last_scan_time (string) the last scan timestamp of the index

metadata.num_replica (number) the index replica count

metadata.stats.last_known_scan_time (number) The index last scan time from indexer in Epoch/UNIX  format

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